In the second half of this morning's activities with the 3-4 graders, we discovered some things about light and telescopes. I handed out diffraction gratings and we looked at the spectrum of the Sun and of the fluorescent lights in the room, discovering that white light is actually composed of many colors. We also looked at discharge tubes filled with different elements, with mercury and helium being the stars. We found that each element emits a unique "fingerprint" of spectral lines. To see a great 2-minute video of everything the kids saw, check this out. This is how we know what stars and other planets are made of.
We then discussed how the colors always appear in a certain order in a rainbow or a diffraction grating: red, orange, yellow, green, blue, violet. Could there be any light which appears before red? Yes, it's called infrared, and we can build cameras to see it even though our eyes can't. I showed this nice video demonstrating the properties of infrared light. Could there be any light which appears after violet? Yes, ultraviolet, and after that would be X-rays and finally gamma rays. We talked about X-rays for a while because some kids were worried about it being dangerous. (Like many other things, they are safe if used properly, but dangerous if not. A yearly dental X-ray is ok, but how do we protect the parts of our bodies which don't need to be X-rayed? And how do we protect the workers who administer dozens of X-rays each day?) I extended that discussion to the ultraviolet and sunlight.
All this was a springboard for discussing telescopes, which is one of the last astronomy standards I hadn't covered yet. Specialized telescopes are built to look at all kinds of light, from gamma rays to the infrared and radio. I showed pictures of some of the big telescopes I have used in my research, and that led to all kinds of interesting questions. We ran out of time, so I may start next Friday by answering more telescope questions.
Showing posts with label light/optics. Show all posts
Showing posts with label light/optics. Show all posts
Friday, May 31, 2013
Saturday, February 16, 2013
Heat, Earth, and Sun
Friday I started earth science with the 5-7 graders at Peregrine
School. We started half an hour late because of the all-school
discussion of the meteor strike over Russia. So I squeezed a lot into
35 minutes before a shortened recess break. We reviewed the structure
of the Earth and then we talked about the three different ways heat
flows: conduction, convection, and radiation (which in this context is
just another word for light; it does not mean ionizing radiation,
which is what you need to protect your DNA from). I brought a torch
and a saucepan to make the discussion of conduction more concrete:
cookware designers want the bottom to conduct heat very well so that
the food is heated evenly, but they want the handle to conduct heat
poorly so that you don't burn yourself. Then I added water to segue
to convection. Because hot fluids rise, convection occurs whenever a
fluid is heated from below, which occurs in very diverse contexts:
boiling water on the stove, fluid rock in Earth's mantle, and the
movement of air in the atmosphere.
Next, I drew a Sun far from our diagram of Earth, and I asked how heat
gets from the Sun to the Earth. It can't be conduction or convection,
because empty space can't do either of these. It's radiation (light).
So we observed thermal radiation (the light emitted by an object by
virtue of its temperature), noting the brightness and color of a light
bulb at different temperatures (achieved by changing the voltage). We
analyzed the color in detail by looking through diffraction gratings
to make rainbows from the white light, and noting which color in the
rainbow was brightest. The pattern that emerges is: raising the
temperature makes the light bluer, and makes it much brighter. We
think of red hot as being about the hottest temperature we ever
encounter, but really white hot is even hotter (the light is a mixture
of red, green, and blue), and blue hot is even hotter than that. (The
ocean and sky are blue because they scatter the blue light from the
Sun, not because they are emitting light.) Even objects at room
temperature emit thermal radiation, but that light is "redder than
red" or infrared. These kids had played with an infrared camera
before, so I didn't bring one, but we discussed their IR camera
experience in this new light. (Read this post to get the basics of
the IR camera experience.)
The last point I made before recess break: Earth's temperature is a
balance between the energy it gets from the Sun and the energy
(infrared light) it emits into space. To maintain a roughly stable
temperature, it must emit as much as it gets. We would examine that
balance in more detail after the break. During the break, I had a
trick to keep them thinking about this subject: I brought a parabolic
mirror, pointed it at the Sun, and we entertained ourselves setting
things on fire.
After the break, before moving on, I felt they needed more practice with
conduction, convection, and radiation, so I had them work in groups to design
thermoses. We put together ideas from the different groups to arrive at a
consensus design which minimizes conduction, convection, and radiation.
Back to the main thread: I noted how the parabolic mirror gathered energy from
the Sun over a largish area and concentrated it on a small area. If we
measured the power (energy per second) falling over one square meter
(about twice the area of the mirror), we would find that it's about
one kilowatt, or 1 kW. I brought a 1 kW hair dryer to make that more
concrete. We then talked about night vs day, and how the Sun is
fairly low in the sky during part of the day, and concluded that the
average power from the Sun on 1 square meter of Earth would be more
like 300 W. So each square meter of Earth should emit about 300 W of
infrared light in order to maintain a stable temperature.
Recall that power emitted ("brightness") increases strongly as the
temperature of an object increases. So if the temperature of that
square meter of Earth is low, it will emit less than it absorbs, and
that will raise its temperature. But if the temperature goes up very
high, it will emit more than it absorbs, and the temp will come down.
We ought to be able to calculate the temp which is just right so that
it emits exactly 300 W. This is where we returned to the computer
programming that the kids are loving so much. Most of these kids are
not familiar with algebra, but they can (with lots of guidance from
me) write a loop over a range of plausible temperatures and print out
the power emitted at each temperature.
To do this, I had to give them the equation for power (in watts)
emitted as a function of temperature: 0.0000000567 T4, where T is in
Kelvins. That led to a discussion of Fahrenheit vs Celsius vs Kelvin.
Fahrenheit is defined so that water freezes at 32 degrees and boils at
212, a 180-degree difference; Celsius is defined so that water freezes
at 0 degrees and boils at 100. Therefore, each Celsius degree is
"bigger" by 180/100 or 9/5. Therefore Fahrenheit = 9/5 Celsius + 32.
Kelvin = Celsius + 273 (I explained about absolute zero), so
Fahrenheit = 9/5 (Kelvin-273) + 32. Admittedly, most students didn't
follow all these steps, but at least one did, and I told the others to
just use this to convert while focusing on the logical steps needed to
carry out their program.
So each group wrote a Python script to check from 1 to 1000 Kelvins,
at each step printing out the power emitted and the Fahrenheit
temperature. It turns out that 26 F is the right temperature for 300
W. Is this a reasonable answer? We discussed the approximations
involved (primarily albedo, using snow as an example). Then we tried
representing this information graphically. Instead of scanning a list
of numbers to find the right temperature, I taught them how to make a
graph of power emitted vs temperature. We then added a horizontal
line at 300 W, and the temp at which the line intersects the curve is
the "right" temp. I really want to work on graph-making and
-interpreting skills, so we discussed the labels we should put on each
axis, and how to summarize the plot in words.
As a teaser for next week, a slightly more rigorous calculation shows
that Earth's global average temperature should be even colder than 26
F. The reason we are not in fact that cold is that our atmosphere
intercepts some of the outgoing infrared light and turns it back to
the surface: the greenhouse effect. There is a natural greenhouse
effect which makes our planet livable. The kids had of course heard
of the greenhouse effect and global warming, so they were able to see
right away that the problem is not the greenhouse effect per se; it is
that we are adding to the natural greenhouse effect, resulting in too
much of a good thing. More on that next week.
The original plot we made:
and a zoom in to the important part:
School. We started half an hour late because of the all-school
discussion of the meteor strike over Russia. So I squeezed a lot into
35 minutes before a shortened recess break. We reviewed the structure
of the Earth and then we talked about the three different ways heat
flows: conduction, convection, and radiation (which in this context is
just another word for light; it does not mean ionizing radiation,
which is what you need to protect your DNA from). I brought a torch
and a saucepan to make the discussion of conduction more concrete:
cookware designers want the bottom to conduct heat very well so that
the food is heated evenly, but they want the handle to conduct heat
poorly so that you don't burn yourself. Then I added water to segue
to convection. Because hot fluids rise, convection occurs whenever a
fluid is heated from below, which occurs in very diverse contexts:
boiling water on the stove, fluid rock in Earth's mantle, and the
movement of air in the atmosphere.
Next, I drew a Sun far from our diagram of Earth, and I asked how heat
gets from the Sun to the Earth. It can't be conduction or convection,
because empty space can't do either of these. It's radiation (light).
So we observed thermal radiation (the light emitted by an object by
virtue of its temperature), noting the brightness and color of a light
bulb at different temperatures (achieved by changing the voltage). We
analyzed the color in detail by looking through diffraction gratings
to make rainbows from the white light, and noting which color in the
rainbow was brightest. The pattern that emerges is: raising the
temperature makes the light bluer, and makes it much brighter. We
think of red hot as being about the hottest temperature we ever
encounter, but really white hot is even hotter (the light is a mixture
of red, green, and blue), and blue hot is even hotter than that. (The
ocean and sky are blue because they scatter the blue light from the
Sun, not because they are emitting light.) Even objects at room
temperature emit thermal radiation, but that light is "redder than
red" or infrared. These kids had played with an infrared camera
before, so I didn't bring one, but we discussed their IR camera
experience in this new light. (Read this post to get the basics of
the IR camera experience.)
The last point I made before recess break: Earth's temperature is a
balance between the energy it gets from the Sun and the energy
(infrared light) it emits into space. To maintain a roughly stable
temperature, it must emit as much as it gets. We would examine that
balance in more detail after the break. During the break, I had a
trick to keep them thinking about this subject: I brought a parabolic
mirror, pointed it at the Sun, and we entertained ourselves setting
things on fire.
After the break, before moving on, I felt they needed more practice with
conduction, convection, and radiation, so I had them work in groups to design
thermoses. We put together ideas from the different groups to arrive at a
consensus design which minimizes conduction, convection, and radiation.
Back to the main thread: I noted how the parabolic mirror gathered energy from
the Sun over a largish area and concentrated it on a small area. If we
measured the power (energy per second) falling over one square meter
(about twice the area of the mirror), we would find that it's about
one kilowatt, or 1 kW. I brought a 1 kW hair dryer to make that more
concrete. We then talked about night vs day, and how the Sun is
fairly low in the sky during part of the day, and concluded that the
average power from the Sun on 1 square meter of Earth would be more
like 300 W. So each square meter of Earth should emit about 300 W of
infrared light in order to maintain a stable temperature.
Recall that power emitted ("brightness") increases strongly as the
temperature of an object increases. So if the temperature of that
square meter of Earth is low, it will emit less than it absorbs, and
that will raise its temperature. But if the temperature goes up very
high, it will emit more than it absorbs, and the temp will come down.
We ought to be able to calculate the temp which is just right so that
it emits exactly 300 W. This is where we returned to the computer
programming that the kids are loving so much. Most of these kids are
not familiar with algebra, but they can (with lots of guidance from
me) write a loop over a range of plausible temperatures and print out
the power emitted at each temperature.
To do this, I had to give them the equation for power (in watts)
emitted as a function of temperature: 0.0000000567 T4, where T is in
Kelvins. That led to a discussion of Fahrenheit vs Celsius vs Kelvin.
Fahrenheit is defined so that water freezes at 32 degrees and boils at
212, a 180-degree difference; Celsius is defined so that water freezes
at 0 degrees and boils at 100. Therefore, each Celsius degree is
"bigger" by 180/100 or 9/5. Therefore Fahrenheit = 9/5 Celsius + 32.
Kelvin = Celsius + 273 (I explained about absolute zero), so
Fahrenheit = 9/5 (Kelvin-273) + 32. Admittedly, most students didn't
follow all these steps, but at least one did, and I told the others to
just use this to convert while focusing on the logical steps needed to
carry out their program.
So each group wrote a Python script to check from 1 to 1000 Kelvins,
at each step printing out the power emitted and the Fahrenheit
temperature. It turns out that 26 F is the right temperature for 300
W. Is this a reasonable answer? We discussed the approximations
involved (primarily albedo, using snow as an example). Then we tried
representing this information graphically. Instead of scanning a list
of numbers to find the right temperature, I taught them how to make a
graph of power emitted vs temperature. We then added a horizontal
line at 300 W, and the temp at which the line intersects the curve is
the "right" temp. I really want to work on graph-making and
-interpreting skills, so we discussed the labels we should put on each
axis, and how to summarize the plot in words.
As a teaser for next week, a slightly more rigorous calculation shows
that Earth's global average temperature should be even colder than 26
F. The reason we are not in fact that cold is that our atmosphere
intercepts some of the outgoing infrared light and turns it back to
the surface: the greenhouse effect. There is a natural greenhouse
effect which makes our planet livable. The kids had of course heard
of the greenhouse effect and global warming, so they were able to see
right away that the problem is not the greenhouse effect per se; it is
that we are adding to the natural greenhouse effect, resulting in too
much of a good thing. More on that next week.
The original plot we made:
Friday, June 1, 2012
Focus, kids!
The Primaria kids are learning about solar ovens; they fried an egg
yesterday and they baked cookies today. I brought in a neat piece of
equipment to complement that: a parabolic mirror about 24" in
diameter. If you point it at the Sun and put anything flammable at
the focus, it will burst into flame in about one second.
I also brought a larger flat mirror and started by having the kids try
to figure out how to set a piece of paper on fire. (I knew they
wouldn't actually be able to do it without guidance, so there were no
safety issues at this stage.) After watching what they did, we
discussed some very basic physics:
--you can't have too many people crowding around the mirror, just as you
wouldn't put the mirror under a tree
--you want to point the mirror at the Sun
So I pointed the larger flat mirror at the Sun and had them try again.
When they couldn't, I pointed out how the "small" mirror was curved
and explained why that was important: light hitting the edge of the
mirror will be bounced toward a point above the center, and light
hitting the center will also be be bounced toward a point above the
center. All the light meets at one point!
With that in mind, I had the kids form a line and I helped them ignite
paper one by one for safety reasons (most of them were also too scared
to try by themselves). I showed how putting the paper directly on the
mirror was no different than just putting the paper in regular
sunlight. As we lifted the paper above the surface of the mirror we
could see the light start to focus down on a smaller area of the
paper. At just the right distance from the mirror, all the light is
focused on such a small area of the paper that it starts to smoke!
This one-by-one part of the activity took most of the time. There was
little or no time for many of the add-ons I had imagined, such as
connecting with the idea of a magnifying glass, discussing how to
store solar energy so you can use it at night, relating to the
temperatures of different planets (concentrating the light mimics the
effect of being closer to the Sun), and having them draw how it works.
With some of the groups, I had brief discussions of some of these
ideas, but we definitely could have used more than the allotted 20
minutes. I especially wish we could have done the drawings...it would
be really interesting to see what the kids would come up with.
I wore pretty dark sunglasses for this activity. At one point I took
my sunglasses off to lend them to a child, and I was just about
blinded...I did have a blind spot for about 5 minutes afterward. So I
would recommend bringing sunglasses for the kids too.
You don't necessarily need a mirror as large as the one I was able to borrow. I've seen survival shows where people used parabolic mirrors as small as the ones in flashlights to start fires. It just requires more patience, and more fine motor skills to hit that small focus.
If you want to add a little more physics, note that the Sun provides over one kilowatt per square meter (the "solar constant" is 1.361 kW/m2, but the atmosphere takes a bit of that). Kids in the upper elementary grades should be able to figure out how many watts their mirror collects and compare that with the power of their microwave oven, the energy consumption of a typical house, etc.
yesterday and they baked cookies today. I brought in a neat piece of
equipment to complement that: a parabolic mirror about 24" in
diameter. If you point it at the Sun and put anything flammable at
the focus, it will burst into flame in about one second.
I also brought a larger flat mirror and started by having the kids try
to figure out how to set a piece of paper on fire. (I knew they
wouldn't actually be able to do it without guidance, so there were no
safety issues at this stage.) After watching what they did, we
discussed some very basic physics:
--you can't have too many people crowding around the mirror, just as you
wouldn't put the mirror under a tree
--you want to point the mirror at the Sun
So I pointed the larger flat mirror at the Sun and had them try again.
When they couldn't, I pointed out how the "small" mirror was curved
and explained why that was important: light hitting the edge of the
mirror will be bounced toward a point above the center, and light
hitting the center will also be be bounced toward a point above the
center. All the light meets at one point!
With that in mind, I had the kids form a line and I helped them ignite
paper one by one for safety reasons (most of them were also too scared
to try by themselves). I showed how putting the paper directly on the
mirror was no different than just putting the paper in regular
sunlight. As we lifted the paper above the surface of the mirror we
could see the light start to focus down on a smaller area of the
paper. At just the right distance from the mirror, all the light is
focused on such a small area of the paper that it starts to smoke!
This one-by-one part of the activity took most of the time. There was
little or no time for many of the add-ons I had imagined, such as
connecting with the idea of a magnifying glass, discussing how to
store solar energy so you can use it at night, relating to the
temperatures of different planets (concentrating the light mimics the
effect of being closer to the Sun), and having them draw how it works.
With some of the groups, I had brief discussions of some of these
ideas, but we definitely could have used more than the allotted 20
minutes. I especially wish we could have done the drawings...it would
be really interesting to see what the kids would come up with.
I wore pretty dark sunglasses for this activity. At one point I took
my sunglasses off to lend them to a child, and I was just about
blinded...I did have a blind spot for about 5 minutes afterward. So I
would recommend bringing sunglasses for the kids too.
You don't necessarily need a mirror as large as the one I was able to borrow. I've seen survival shows where people used parabolic mirrors as small as the ones in flashlights to start fires. It just requires more patience, and more fine motor skills to hit that small focus.
If you want to add a little more physics, note that the Sun provides over one kilowatt per square meter (the "solar constant" is 1.361 kW/m2, but the atmosphere takes a bit of that). Kids in the upper elementary grades should be able to figure out how many watts their mirror collects and compare that with the power of their microwave oven, the energy consumption of a typical house, etc.
Friday, March 2, 2012
Kindergarten Energy
Today I discussed energy with the pre-K/K kids. I followed the same basic plan as I did when I discussed energy with the elementary kids (minus the last three paragraphs). I know they've studied the water cycle quite a bit, so at the end I related it to the water cycle: water in the clouds has potential energy, water in the river has energy of motion, damming the river stores the energy, letting water our of the dam turns a turbine which generates electricity, etc.
In the remaining time, instead of having the kids draw pictures of different forms of energy as I did with the elementary kids, I let them play with the lights, prisms and lenses which were so popular last week. The primaria kids loved them too. They were very disappointed when science time was up; in fact, one of them cried so much that I decided to leave all the materials at the school so that they could play with them in afterschool care as well. This is a good thing for the future of science: girls crying for more science time! Apparently this activity was a big hit in aftercare as well.
In the remaining time, instead of having the kids draw pictures of different forms of energy as I did with the elementary kids, I let them play with the lights, prisms and lenses which were so popular last week. The primaria kids loved them too. They were very disappointed when science time was up; in fact, one of them cried so much that I decided to leave all the materials at the school so that they could play with them in afterschool care as well. This is a good thing for the future of science: girls crying for more science time! Apparently this activity was a big hit in aftercare as well.
Sunday, February 26, 2012
Escape from Prism
Last Friday the elementary kids and I had one of the best experiences
ever! They were so engaged it was amazing, and a pleasure to watch.
The plan was to build on what we learned last time---that light is a
form of energy---by learning more about light. (Grades 4-6 had
already seen some of the following because of the mixed-up schedule in
January, but I adjusted accordingly. It turns out that review is
good; having already seen a concept in action doesn't automatically
lead to being able to think clearly about it later! And this was the
first time grades 1-3 were seeing this.)
I started by handing out diffraction gratings, which spread out light
into its component wavelengths, also known as colors. We looked at
different sources of light: a fluorescent bulb which has a limited
number of very specific wavelengths (a picture of which can be seen here);
an incandescent bulb which emits all shades of colors continuously
blending into each other like a rainbow; and the sunlight coming
through a crack in my meticulously placed window shades. This
provided the opportunity to emphasize a few things about doing
experiments. First, the power of careful observation. At first, kids
just exclaimed "rainbow" for everything. The more observant ones saw
that the fluorescent light was different, and by praising them I
motivated everyone to pay more attention. Second, the need to
eliminate extraneous factors when experimenting. If you look through
a diffraction grating in a normal room, you will see lots of rainbows
coming from light reflected from the ceiling lights, from sunlight
through the windows, etc. As I gradually eliminated these extraneous
factors, they were able to see how the experiment made more and more
sense.
The bulbs also gave me a chance to talk mention energy. The
fluorescent bulb saves energy by emitting only a few specific colors,
which are carefully designed to appear as pleasing a white as possible
when mixed. An incandescent bulb wastes energy by emitting all
possible wavelengths, including some that the eye can't even see.
Because the fluorescent bulb emits energy just in the form of visible
light, it is not very hot to the touch, whereas the incandescent bulb
is extremely hot to the touch.
We reviewed the order of the colors in the rainbow, and I wrote them
on the board in order. Then I asked if there could be anything
before violet or after red; maybe a color our eyes can't see? (I got
a lot of funny answers to this question: brown and white were
suggested as colors we can't see!) Grades 4-6 needed some prodding,
but did come up with ultraviolet and infrared because they had learned
about them before. With grades 1-3 I focused on reassuring them that
these weren't magical things but just colors that our eyes weren't
sensitive to, and that they had a place in this very specific order
that they had memorized through song. We talked about ultraviolet in
relation to sunburn and the need for sunblock in the summer.
And now comes the exciting part! Thanks to Vera I had just gotten
hold of colored light sources which are perfect for kids experimenting
with mixing colors. I gave each group of 2-3 kids a set of red,
green, and blue lights, and asked them to figure out how to make
white. They were really engaged in this and in all that follows.
When they figured out who to make white (mix all three equally), I
asked them to figure out how to make black! This led them to realize
themselves that black is not a color of light, but the absence of
light. I asked them whether mixing read, green, and blue paint would
yield white, and they had enough experience with this to say no, it
would yield something very dark. So I asked them why green paint
looks green, or a green plant looks green. I drew red+green+blue
light coming from the Sun and hitting a plant, and I drew an eye
looking at the plant. What should I draw going from the plant to the
eye? Only green. So the red and blue got absorbed, and green got reflected.
The plant subtracts the red and blue from the total amount
of light. Similarly, paint subtracts from the light that hits it, so
that mixing all colors of paint will subtract all colors and leave you
with something very dark.
Although they were really engaged in this, it gets even better. The
light sources have optional caps which let light out just through a
slit so that it really looks like a ray of light. You can set the
light on the table and really see the ray of light on the table. With
those one, I passed out prisms and lenses so they could see how these
affect light. You can really see rays of light bent by the prism and
focused by the lens! (This is a special kind of lens which is just a
"slice" of a lens designed to lay flat on the table.) The kids went
crazy coming up with different combinations of lenses and prisms which
make the light go different ways; rotating prisms, etc. The cool
factor was increased by the fact that the kids thought the light with
a slit on it was a laser! I also brought out some glasses of water,
so they could see how water bends light as well. The kids were so
engaged in this that half of them stayed in the science room through
recess! With twice as many lights and prisms and lenses per kid, they
came up with some really elaborate creations:
A few kids asked me if they could take the lights home. They
couldn't, but parents can order them pretty cheaply here.
Mouse over the little pictures to see the light-ray behavior I
described. We will also work on getting a few for the school so that
kids can play with them during free choice time.
ever! They were so engaged it was amazing, and a pleasure to watch.
The plan was to build on what we learned last time---that light is a
form of energy---by learning more about light. (Grades 4-6 had
already seen some of the following because of the mixed-up schedule in
January, but I adjusted accordingly. It turns out that review is
good; having already seen a concept in action doesn't automatically
lead to being able to think clearly about it later! And this was the
first time grades 1-3 were seeing this.)
I started by handing out diffraction gratings, which spread out light
into its component wavelengths, also known as colors. We looked at
different sources of light: a fluorescent bulb which has a limited
number of very specific wavelengths (a picture of which can be seen here);
an incandescent bulb which emits all shades of colors continuously
blending into each other like a rainbow; and the sunlight coming
through a crack in my meticulously placed window shades. This
provided the opportunity to emphasize a few things about doing
experiments. First, the power of careful observation. At first, kids
just exclaimed "rainbow" for everything. The more observant ones saw
that the fluorescent light was different, and by praising them I
motivated everyone to pay more attention. Second, the need to
eliminate extraneous factors when experimenting. If you look through
a diffraction grating in a normal room, you will see lots of rainbows
coming from light reflected from the ceiling lights, from sunlight
through the windows, etc. As I gradually eliminated these extraneous
factors, they were able to see how the experiment made more and more
sense.
The bulbs also gave me a chance to talk mention energy. The
fluorescent bulb saves energy by emitting only a few specific colors,
which are carefully designed to appear as pleasing a white as possible
when mixed. An incandescent bulb wastes energy by emitting all
possible wavelengths, including some that the eye can't even see.
Because the fluorescent bulb emits energy just in the form of visible
light, it is not very hot to the touch, whereas the incandescent bulb
is extremely hot to the touch.
We reviewed the order of the colors in the rainbow, and I wrote them
on the board in order. Then I asked if there could be anything
before violet or after red; maybe a color our eyes can't see? (I got
a lot of funny answers to this question: brown and white were
suggested as colors we can't see!) Grades 4-6 needed some prodding,
but did come up with ultraviolet and infrared because they had learned
about them before. With grades 1-3 I focused on reassuring them that
these weren't magical things but just colors that our eyes weren't
sensitive to, and that they had a place in this very specific order
that they had memorized through song. We talked about ultraviolet in
relation to sunburn and the need for sunblock in the summer.
And now comes the exciting part! Thanks to Vera I had just gotten
hold of colored light sources which are perfect for kids experimenting
with mixing colors. I gave each group of 2-3 kids a set of red,
green, and blue lights, and asked them to figure out how to make
white. They were really engaged in this and in all that follows.
When they figured out who to make white (mix all three equally), I
asked them to figure out how to make black! This led them to realize
themselves that black is not a color of light, but the absence of
light. I asked them whether mixing read, green, and blue paint would
yield white, and they had enough experience with this to say no, it
would yield something very dark. So I asked them why green paint
looks green, or a green plant looks green. I drew red+green+blue
light coming from the Sun and hitting a plant, and I drew an eye
looking at the plant. What should I draw going from the plant to the
eye? Only green. So the red and blue got absorbed, and green got reflected.
The plant subtracts the red and blue from the total amount
of light. Similarly, paint subtracts from the light that hits it, so
that mixing all colors of paint will subtract all colors and leave you
with something very dark.
Although they were really engaged in this, it gets even better. The
light sources have optional caps which let light out just through a
slit so that it really looks like a ray of light. You can set the
light on the table and really see the ray of light on the table. With
those one, I passed out prisms and lenses so they could see how these
affect light. You can really see rays of light bent by the prism and
focused by the lens! (This is a special kind of lens which is just a
"slice" of a lens designed to lay flat on the table.) The kids went
crazy coming up with different combinations of lenses and prisms which
make the light go different ways; rotating prisms, etc. The cool
factor was increased by the fact that the kids thought the light with
a slit on it was a laser! I also brought out some glasses of water,
so they could see how water bends light as well. The kids were so
engaged in this that half of them stayed in the science room through
recess! With twice as many lights and prisms and lenses per kid, they
came up with some really elaborate creations:
A few kids asked me if they could take the lights home. They
couldn't, but parents can order them pretty cheaply here.
Mouse over the little pictures to see the light-ray behavior I
described. We will also work on getting a few for the school so that
kids can play with them during free choice time.
Saturday, February 11, 2012
Energy
The elementary school has now moved into its new site, and Friday I
taught in a bona fide science room for the first time! It was good to
see the kids in grades 1-3, whom I hadn't seen in a while, and Teacher
Cara asked to try a new way of splitting into groups: instead of three
groups of seven, I would be with the seven in grades 4-6 for half an
hour, then the 14 in grades 1-3 for half an hour. This gave me more
time with each group, which I believe was very useful as described
below. And I was pleasantly surprised at the manageability of this
large group; the more formal school setting seems to have drawn out more
serious behavior.
The students are starting a big unit on energy. As planned by Lorie,
the emphasis is on ecology: how all living things get their energy
ultimately from the Sun, how it is passed from plants to plant-eaters
to carnivores, and how we can take advantage of the Sun's energy more
directly by building things like solar ovens. I plan to come in and
do the more physics-y aspects.
So Friday I started with an overview of different forms of energy. I
elicited kids' ideas about energy and the forms it comes in, so we
covered these concepts as they came up, in a different order in each
group:
--energy of motion is called kinetic energy
--the energy in food is called chemical energy and is no different
from the chemical energy in gasoline ("how did you get to school
today?") or coal/gas-burning power plants ("where does the
electricity in the wall come from?"). I burned a few chemicals as a
demonstration: first a potato chip, which burns quite vigorously,
revealing it has a lot of chemical energy (a chance to slip in a few
words of nutrition advice); a piece of whole-wheat spaghetti, which
burns much less vigorously (it may not have that much less energy,
but it is released more slowly, which is good for their bodies); a
candle; and alcohol. For the candle I made an analogy to the human
body: your body is slowly burning the food, so you don't get very
hot but your body is warm to the touch. The alcohol was a hit; it
burns so cleanly that you can't see any smoke or flame, but you can
really feel the heat. (Advice to food-burners: it's much harder
than it looks. You should practice everything at home. Few foods
really ignite, even some, like sugar, which you would think would be
easy. I can't get alcohol to ignite with a lighter, so I bring a
torch which impresses the kids. Thus the alcohol should be in a
bowl rather than a glass, so the torch can reach it. Etc. You
really have to practice!)
The flame gives off two other kinds of energy:
--heat. Our bodies turn chemical energy into energy of motion and
heat. What are some ways you can turn heat energy into other forms
of energy? (A hot air balloon, perhaps.)
--light. These kids were very surprisingly familiar with the concept
that light is a form of energy; it turned out that Lorie had already
discussed that with them. But even so, I had a nice demo that light
can push on things and start them in motion, using a very simple
device called a Crooke's radiometer. Shining light on a Crooke's
radiometer causes it to spin, which demonstrates the conversion of
light energy into energy of motion. (Maven alert: this conversion
is not really in one step, but I glossed over that for the sake of a
good demo.) The solar oven they will build will demonstrate the
conversion of light to heat.
--potential energy. A ball on the edge of a desk has the potential to
fall and gain kinetic energy, so it has potential energy from
gravity. Another example is a stretched rubber band: it has the
potential, if I let go, to go flying off somewhere.
--electricity. Plugging the light into the wall to spin the
radiometer was a clear demonstration of the conversion of
electricity into light, which suggests that electricity is a form of
energy. I asked them where it comes from, which led to interesting
discussions. If they suggest solar panels, that's a chance to
highlight the conversion of light to another form of energy (and a
chance to disabuse them of the notion that most of our power is
clean). Fuel-burning power plants convert chemical energy into
electricity. Hydro plants convert gravitational potential energy
into electricity.
I led them to many different examples of one form of energy converting
to another. For example, a red-hot electric stove burner converts
some of its heat energy into light. I swung a pendulum from my
finger. A pendulum converts back and forth between potential and
kinetic but eventually loses them both. Where did the energy go?
Into heat, by stirring up the air in the room ever so slightly and by
rubbing my finger. How do you feel when exercising vigorously? Hot!
So your body also "loses" energy by converting some to heat which
escapes. It turns out that you never really lose energy when doing
these conversions, but heat energy is only useful if it's
concentrated, like right around a flame. Once it's spread out, it's
still there but difficult to capture and make use of. That's why
people say we "use up" energy when, in a strict physics sense, we
don't.
With all these different kinds of energy, I wanted to come back to a
unifying theme. (The unifying themes of science tend to get lost when the
details are lost, which is a shame because the unity of scientific knowledge is
a beautiful thing.) The fact that we can do all these conversions means that
all these things are, down deep, just different manifestations of the
same thing: energy. A good analogy is money: we can convert a dollar
bill into four quarters, or ten dimes, or two quarters and five dimes,
etc, and these things all look different but are really the same
thing. (For adults we could also mention stocks, bonds, derivatives,
credit default swaps, etc.)
In the last ten minutes, the ten minutes I never had with the old
schedule, I had the kids use their creativity to think of and draw as
many energy-converting devices as they could think of. The more and
the more unexpected, the better, just as in a Rube Goldberg machine.
This turned out very well; I had a chance to circulate and consult
with each child one-on-one, and they had a chance to put the new
concepts into practice using familiar skills and choosing their own
focus. Here are some results:
taught in a bona fide science room for the first time! It was good to
see the kids in grades 1-3, whom I hadn't seen in a while, and Teacher
Cara asked to try a new way of splitting into groups: instead of three
groups of seven, I would be with the seven in grades 4-6 for half an
hour, then the 14 in grades 1-3 for half an hour. This gave me more
time with each group, which I believe was very useful as described
below. And I was pleasantly surprised at the manageability of this
large group; the more formal school setting seems to have drawn out more
serious behavior.
The students are starting a big unit on energy. As planned by Lorie,
the emphasis is on ecology: how all living things get their energy
ultimately from the Sun, how it is passed from plants to plant-eaters
to carnivores, and how we can take advantage of the Sun's energy more
directly by building things like solar ovens. I plan to come in and
do the more physics-y aspects.
So Friday I started with an overview of different forms of energy. I
elicited kids' ideas about energy and the forms it comes in, so we
covered these concepts as they came up, in a different order in each
group:
--energy of motion is called kinetic energy
--the energy in food is called chemical energy and is no different
from the chemical energy in gasoline ("how did you get to school
today?") or coal/gas-burning power plants ("where does the
electricity in the wall come from?"). I burned a few chemicals as a
demonstration: first a potato chip, which burns quite vigorously,
revealing it has a lot of chemical energy (a chance to slip in a few
words of nutrition advice); a piece of whole-wheat spaghetti, which
burns much less vigorously (it may not have that much less energy,
but it is released more slowly, which is good for their bodies); a
candle; and alcohol. For the candle I made an analogy to the human
body: your body is slowly burning the food, so you don't get very
hot but your body is warm to the touch. The alcohol was a hit; it
burns so cleanly that you can't see any smoke or flame, but you can
really feel the heat. (Advice to food-burners: it's much harder
than it looks. You should practice everything at home. Few foods
really ignite, even some, like sugar, which you would think would be
easy. I can't get alcohol to ignite with a lighter, so I bring a
torch which impresses the kids. Thus the alcohol should be in a
bowl rather than a glass, so the torch can reach it. Etc. You
really have to practice!)
The flame gives off two other kinds of energy:
--heat. Our bodies turn chemical energy into energy of motion and
heat. What are some ways you can turn heat energy into other forms
of energy? (A hot air balloon, perhaps.)
--light. These kids were very surprisingly familiar with the concept
that light is a form of energy; it turned out that Lorie had already
discussed that with them. But even so, I had a nice demo that light
can push on things and start them in motion, using a very simple
device called a Crooke's radiometer. Shining light on a Crooke's
radiometer causes it to spin, which demonstrates the conversion of
light energy into energy of motion. (Maven alert: this conversion
is not really in one step, but I glossed over that for the sake of a
good demo.) The solar oven they will build will demonstrate the
conversion of light to heat.
--potential energy. A ball on the edge of a desk has the potential to
fall and gain kinetic energy, so it has potential energy from
gravity. Another example is a stretched rubber band: it has the
potential, if I let go, to go flying off somewhere.
--electricity. Plugging the light into the wall to spin the
radiometer was a clear demonstration of the conversion of
electricity into light, which suggests that electricity is a form of
energy. I asked them where it comes from, which led to interesting
discussions. If they suggest solar panels, that's a chance to
highlight the conversion of light to another form of energy (and a
chance to disabuse them of the notion that most of our power is
clean). Fuel-burning power plants convert chemical energy into
electricity. Hydro plants convert gravitational potential energy
into electricity.
I led them to many different examples of one form of energy converting
to another. For example, a red-hot electric stove burner converts
some of its heat energy into light. I swung a pendulum from my
finger. A pendulum converts back and forth between potential and
kinetic but eventually loses them both. Where did the energy go?
Into heat, by stirring up the air in the room ever so slightly and by
rubbing my finger. How do you feel when exercising vigorously? Hot!
So your body also "loses" energy by converting some to heat which
escapes. It turns out that you never really lose energy when doing
these conversions, but heat energy is only useful if it's
concentrated, like right around a flame. Once it's spread out, it's
still there but difficult to capture and make use of. That's why
people say we "use up" energy when, in a strict physics sense, we
don't.
With all these different kinds of energy, I wanted to come back to a
unifying theme. (The unifying themes of science tend to get lost when the
details are lost, which is a shame because the unity of scientific knowledge is
a beautiful thing.) The fact that we can do all these conversions means that
all these things are, down deep, just different manifestations of the
same thing: energy. A good analogy is money: we can convert a dollar
bill into four quarters, or ten dimes, or two quarters and five dimes,
etc, and these things all look different but are really the same
thing. (For adults we could also mention stocks, bonds, derivatives,
credit default swaps, etc.)
In the last ten minutes, the ten minutes I never had with the old
schedule, I had the kids use their creativity to think of and draw as
many energy-converting devices as they could think of. The more and
the more unexpected, the better, just as in a Rube Goldberg machine.
This turned out very well; I had a chance to circulate and consult
with each child one-on-one, and they had a chance to put the new
concepts into practice using familiar skills and choosing their own
focus. Here are some results:
 |
| The drawing above incorporates elements of the classroom (Vaca the rabbit): this is basically a rabbit-petting machine. |
 |
| This one (above) was continued on the back, with the electricity from the hydro plant going to school and people using it. |
 |
| Note the creative elements above: the potential energy of the tub of water will be released when its wooden support burns, and the pinwheels at left are like windmills, but are turned by water flowing downhill. A large amount of potential energy is stored when constructing the machine, by piling dirt on a removable cover over a pit. |
 |
| I like this one because the student considered practical matters. There is a ball return device so the machine can keep going without manual reloading. (I helped the younger students put labels on parts of their drawings.) |
Saturday, February 4, 2012
Seeing the World in a New Light
On Friday I was back at Primaria, building on
what we learned last time about colors of light. I handed out diffraction
gratings again, so they could again see how white light is composed of colors.
(I have my doubts that they really get the message that white light is
composed of colors; what they take away seems to be that the device
makes rainbows appear, which is not at all the same thing. Maybe a
dark room with a single source of light would help.) I asked them to
list the colors in order of appearance, in English and Spanish. I was
surprised to see how much trouble they had listing them in order of
appearance on the diffraction grating; they were all over the map. One
idea for the future is to have them draw what they see. But I was
even more surprised when they burst out in song "Red, orange, yellow /
Green, blue purple" when they realized that I was essentially asking
for the colors of the rainbow. They had memorized this song which
perfectly described what they were unable to describe when asked about
their observations! There must be something I can learn from this,
but I'm not sure what.
I wrote the names of the colors in this order on the board, so that
"range of colors" had a more concrete meaning. Then I asked them if
there could be anything "before" red or "after" purple (aka violet).
One boy in one group actually guessed infrared, presumably because he
had overheard me use that word to a teacher. I briefly mentioned
ultraviolet to connect it with the need for sunscreen in summer, but
the main purpose was to show the infrared camera. Things at room
temperature shine in infrared light, and the warmer they are the more
they shine, so the infrared camera is really a very different way of
looking at the world. You can walk into a kitchen, as we did, and be
blinded by the water boiling on the stove. A hand holding an ice cube
looks like a bright silhouette with a black rectangle in front of it.
People glow...you can see the glow radiating out from the openings in
their clothing, and from under their hair. You can make your way in
the dark, because everything is glowing to some extent. You can find
people hiding in the dark.
One group insisted they could see in the dark with their own eyes, so
I took them into the only room in the school which has no window: the
adult bathroom. (Linus had previously voiced this "I can see in the
dark" meme with great confidence, so I was sure it was something that
kid in the school bragged about and needed to be refuted.) I was
pretty sure that when we got in there and shut the door, most kids
would be scared and admit that they couldn't see in the dark. That
happened to one child, but several others were perfectly content with
the light coming from the gap under the door and said, see, we can see
in the dark. Preconceptions are very difficult to uproot! I
discussed this with Linus later and we agreed that he could see when
it was kind of dark, but not when it was perfectly dark. So
terminology matters. Given a way to save face, I got him to agree
with my scientific conclusion; but I had to give him a way to save
face. In any case, the little bit we could see in the mostly-dark
bathroom was nothing compared to how well we could see with the
infrared camera!
I also showed how the IR camera could see through some things which
are opaque to visible light (eg, a black plastic garbage bag, and I
mentioned but did not demonstrate smoke), and others are opaque to IR
but transparent in visible light (eg, glass windows; when I pointed
the camera at the window we saw only my reflection on the camera
display). Outside, the sky is black even during the day because the
air is relatively cold.
If all this sounds interesting, I recommend you watch this video to
get an idea of what the world looks like in the infrared. Meanwhile,
I have to make some notes for future demos: (1) it was hard for
everyone even in a group of seven to see the display. I thought I had
solved this problem by bringing a laptop and not relying on just the
camera's small screen, but you cannot imagine how seven kids have
trouble seeing the same laptop screen at once. In addition, the latop
screen was small enough that it didn't really grab their
attention. Next time, I will either bring a large LCD display, or use
a projector. (2) This activity is not very hands-on. The kids had
trouble staying tuned in. It would be good if I had the camera on a
fixed tripod out of their reach, set up a giant display with the
projector, and just let them do things and bring things in front of
the camera.
what we learned last time about colors of light. I handed out diffraction
gratings again, so they could again see how white light is composed of colors.
(I have my doubts that they really get the message that white light is
composed of colors; what they take away seems to be that the device
makes rainbows appear, which is not at all the same thing. Maybe a
dark room with a single source of light would help.) I asked them to
list the colors in order of appearance, in English and Spanish. I was
surprised to see how much trouble they had listing them in order of
appearance on the diffraction grating; they were all over the map. One
idea for the future is to have them draw what they see. But I was
even more surprised when they burst out in song "Red, orange, yellow /
Green, blue purple" when they realized that I was essentially asking
for the colors of the rainbow. They had memorized this song which
perfectly described what they were unable to describe when asked about
their observations! There must be something I can learn from this,
but I'm not sure what.
I wrote the names of the colors in this order on the board, so that
"range of colors" had a more concrete meaning. Then I asked them if
there could be anything "before" red or "after" purple (aka violet).
One boy in one group actually guessed infrared, presumably because he
had overheard me use that word to a teacher. I briefly mentioned
ultraviolet to connect it with the need for sunscreen in summer, but
the main purpose was to show the infrared camera. Things at room
temperature shine in infrared light, and the warmer they are the more
they shine, so the infrared camera is really a very different way of
looking at the world. You can walk into a kitchen, as we did, and be
blinded by the water boiling on the stove. A hand holding an ice cube
looks like a bright silhouette with a black rectangle in front of it.
People glow...you can see the glow radiating out from the openings in
their clothing, and from under their hair. You can make your way in
the dark, because everything is glowing to some extent. You can find
people hiding in the dark.
One group insisted they could see in the dark with their own eyes, so
I took them into the only room in the school which has no window: the
adult bathroom. (Linus had previously voiced this "I can see in the
dark" meme with great confidence, so I was sure it was something that
kid in the school bragged about and needed to be refuted.) I was
pretty sure that when we got in there and shut the door, most kids
would be scared and admit that they couldn't see in the dark. That
happened to one child, but several others were perfectly content with
the light coming from the gap under the door and said, see, we can see
in the dark. Preconceptions are very difficult to uproot! I
discussed this with Linus later and we agreed that he could see when
it was kind of dark, but not when it was perfectly dark. So
terminology matters. Given a way to save face, I got him to agree
with my scientific conclusion; but I had to give him a way to save
face. In any case, the little bit we could see in the mostly-dark
bathroom was nothing compared to how well we could see with the
infrared camera!
I also showed how the IR camera could see through some things which
are opaque to visible light (eg, a black plastic garbage bag, and I
mentioned but did not demonstrate smoke), and others are opaque to IR
but transparent in visible light (eg, glass windows; when I pointed
the camera at the window we saw only my reflection on the camera
display). Outside, the sky is black even during the day because the
air is relatively cold.
If all this sounds interesting, I recommend you watch this video to
get an idea of what the world looks like in the infrared. Meanwhile,
I have to make some notes for future demos: (1) it was hard for
everyone even in a group of seven to see the display. I thought I had
solved this problem by bringing a laptop and not relying on just the
camera's small screen, but you cannot imagine how seven kids have
trouble seeing the same laptop screen at once. In addition, the latop
screen was small enough that it didn't really grab their
attention. Next time, I will either bring a large LCD display, or use
a projector. (2) This activity is not very hands-on. The kids had
trouble staying tuned in. It would be good if I had the camera on a
fixed tripod out of their reach, set up a giant display with the
projector, and just let them do things and bring things in front of
the camera.
Friday, January 27, 2012
Rainbows keep falling on my head
Yesterday Vera guided the 4-6 graders in exploring how different
colors of light mix. As you may recall, in her previous outing she
showed how light from a white bulb can be split into various colors
like a rainbow. This shows that we perceive as white is actually a
mixture of all colors, so a natural question to explore next is: What
happens if we mix different combinations of colors, like red+green,
green+blue, etc? This is a lot like mixing colors of paint, but not
exactly because many colors of paint mixed together approaches the
appearance of black, but many colors of light mixed together
approaches the appearance of white.
She directed the students to play with this computer simulation which you are free to do at home with your child. After about 5 minutes of just playing
with it, the students were guided through a set of thought-provoking
questions which made them go back and test the hypotheses they had
formed regarding light.
The concepts are similar to many I wrote about in my last visit to Primaria, so I won't belabor them here. I will remark instead on the
importance of testing students' mental models. It's one thing to say
that "white light contains all colors" but it's quite another to probe
what students really think that means in practice. If you put white
light into red glass, did the glass turn all the other colors into
red, or did it just block all the non-red colors from passing through?
The answer is the latter, but the most interesting thing is the
thought process which leads to that. How would we devise an
experiment to distinguish between these two hypotheses? By passing
the resulting red light through blue glass: if the conversion
hypothesis is true, a lot of blue light will come out, but if the
blocking hypothesis is true, basically no light will come out (because
the blue glass will block the red light). It's not enough just to
tell kids that "white light contains all colors." If you tell that to
7 kids, they will have 7 slightly different ideas about what that means
in practice. So a teacher has to find ways for students to test and
modify their mental models. Not only does this lead to a better
mental model, but it make students practice that most important skill:
using evidence to improve their model of how the world works.
colors of light mix. As you may recall, in her previous outing she
showed how light from a white bulb can be split into various colors
like a rainbow. This shows that we perceive as white is actually a
mixture of all colors, so a natural question to explore next is: What
happens if we mix different combinations of colors, like red+green,
green+blue, etc? This is a lot like mixing colors of paint, but not
exactly because many colors of paint mixed together approaches the
appearance of black, but many colors of light mixed together
approaches the appearance of white.
She directed the students to play with this computer simulation which you are free to do at home with your child. After about 5 minutes of just playing
with it, the students were guided through a set of thought-provoking
questions which made them go back and test the hypotheses they had
formed regarding light.
The concepts are similar to many I wrote about in my last visit to Primaria, so I won't belabor them here. I will remark instead on the
importance of testing students' mental models. It's one thing to say
that "white light contains all colors" but it's quite another to probe
what students really think that means in practice. If you put white
light into red glass, did the glass turn all the other colors into
red, or did it just block all the non-red colors from passing through?
The answer is the latter, but the most interesting thing is the
thought process which leads to that. How would we devise an
experiment to distinguish between these two hypotheses? By passing
the resulting red light through blue glass: if the conversion
hypothesis is true, a lot of blue light will come out, but if the
blocking hypothesis is true, basically no light will come out (because
the blue glass will block the red light). It's not enough just to
tell kids that "white light contains all colors." If you tell that to
7 kids, they will have 7 slightly different ideas about what that means
in practice. So a teacher has to find ways for students to test and
modify their mental models. Not only does this lead to a better
mental model, but it make students practice that most important skill:
using evidence to improve their model of how the world works.
Wednesday, January 11, 2012
Infrared Enlightenment
On Tuesday, Vera taught the 4-6 graders. She will be switching in more often in the next months. She's also a physicist and astronomer, and we talk constantly about teaching physics and astronomy, so we're pretty much interchangeable in terms of the experience we provide the children. We both use active learning techniques even in our college classrooms, so with these children we certainly do as much hands-on learning as possible.
Vera started by showing that white light is made of a spectrum of colors, just as I did with the Primaria class last Friday. When we separate the colors, we see a spectrum from violet to red just like in a rainbow, but there are additional "colors" (wavelengths of light) we can't see: ultraviolet (coming before violet in the spectrum) and infrared (coming after red). We have to be aware of ultraviolet because that's what gives us sunburn, but the focus Tuesday was on infrared. Last Friday, I left an infrared videocamera at the school for the children to explore with. It gives a whole new way of seeing things: it basically sees how hot things are, so for example the sky is dark (cold) and people are fairly bright (warm). Infrared light also interacts with matter differently than does visible light. For example, with the infrared camera you may not be able to see through a window which is transparent to visible light, but you may be able to see through a black plastic garbage bag. You may want to watch this short video of all these effects with your child.
I'll be doing infrared with Primaria next week, so I may post more details then. This post is brief because the video linked to above is better than anything I could write here. I definitely encourage you to watch it with your child and discuss what he/she saw and did when he/she had the camera in his/her own hands! In addition to the scientific aspects, there is some value in realizing that the whole world can be looked at in a way so completely different from the way we normally do.
Vera started by showing that white light is made of a spectrum of colors, just as I did with the Primaria class last Friday. When we separate the colors, we see a spectrum from violet to red just like in a rainbow, but there are additional "colors" (wavelengths of light) we can't see: ultraviolet (coming before violet in the spectrum) and infrared (coming after red). We have to be aware of ultraviolet because that's what gives us sunburn, but the focus Tuesday was on infrared. Last Friday, I left an infrared videocamera at the school for the children to explore with. It gives a whole new way of seeing things: it basically sees how hot things are, so for example the sky is dark (cold) and people are fairly bright (warm). Infrared light also interacts with matter differently than does visible light. For example, with the infrared camera you may not be able to see through a window which is transparent to visible light, but you may be able to see through a black plastic garbage bag. You may want to watch this short video of all these effects with your child.
I'll be doing infrared with Primaria next week, so I may post more details then. This post is brief because the video linked to above is better than anything I could write here. I definitely encourage you to watch it with your child and discuss what he/she saw and did when he/she had the camera in his/her own hands! In addition to the scientific aspects, there is some value in realizing that the whole world can be looked at in a way so completely different from the way we normally do.
Saturday, January 7, 2012
Let There Be Light! Part II
You may recall that I was disappointed with the way my light and shadow session with Primaria just before Christmas break turned out,
so I decided to cover some of the same concepts again with a different
approach, as well as add some new concepts related to light. I bought
some powerful light sources from Big 5: an LED lantern and a big
LED flashlight, and I borrowed an old slide projector just to use as a
light source. (Note to self: check eBay for used slide projectors!)
I started by showing how white light is made up of many colors. I had
them look at a compact fluorescent bulb through a special piece of
plastic (a diffraction grating) which separates the different
wavelengths of light. Different wavelengths are perceived as
different colors, so they saw this:
They described it as a rainbow, and yes, it does show that white light
is composed of colors, but it's actually even more interesting than
that. (This part in parentheses I did not go over with the kids, but
it might be useful for other audiences. If the light bulb emitted a
continuous range of wavelengths, the colors would be spread out
continuously and would just be a smear like an actual rainbow is.
This is what an incandescent bulb would look like through a diffraction grating.
But the CF bulb emits only a few specific wavelengths, so you see a
well-defined image of the bulb in one very specific shade of purple,
another well-defined image in a very specific shade of blue, etc.
This is related to how the CF bulb gets its greater energy efficiency.
The incandescent bulb emits a very broad range of wavelengths, from
the ultraviolet to the infrared, and many of these are wasted because
human eyes cannot detect them. The CF bulb by design emits only a few
specific wavelengths which are all chosen to be visible, so none of
its output is wasted. Therefore, for a given amount of visible light
output (listed in lumens on the package), it uses much less energy
(listed as watts on the package). So always buy light bulbs by
looking at the lumens, not watts! You may also see CF packages which
say "100 W equivalent", which means they give off as many lumens as a
100 W incandescent bulb, even while they drain fewer watts from the
grid. And this is also why the light from a CF bulb is sometimes
ugly: engineers have to work hard to build a set of specific
wavelengths which convincingly fake a full range of wavelengths.)
We did talk about what colors might exist beyond violet on one end of
the spectrum and beyond red on the other end. I asked, and none of
them knew that ultraviolet light from the Sun can cause sunburn, so
now they know why their parents slather on sunblock so often! I also
mentioned that next time I will bring an infrared camera so they can
see what the world looks like in that "color."
Having established that white light is composed of many colors, I
asked them about their experience mixing paints of different colors.
What do you get when you mix many colors of paint together? Some of
them knew that you get a yucky dark mess, not white. So mixing colors
is different with light than with paint! (Again, this is above their
age range, but possibly useful to readers: the difference stems from
the fact that red paint, for example, is red because it absorbs all
colors other than red, and reflects red. So if you mix that with
paint which absorbs all colors other than green and paint which
absorbs all colors other than blue, you basically have paint which
absorbs all colors! Paint basically subtracts light rather than adds
light.)
Next, I fired up the slide projector, which I has set up with a small
statute near the screen so that the statue's shadow was clearly
visible. (In practice, I had determined that using a 5-year-old as
the shadowmaker was not practical; it was difficult to keep him still
and to give directions about moving right vs left, etc. I also
darkened the room as much as I could beforehand.) I then turned on a
powerful flashlight and showed how you get two shadows with two light
sources. (This is a powerful argument against Moon-landing-hoax
conspiracy theorists, by the way, who claim that the lighting in the
astronaut's videos came from multiple studio lights; look at the video
and you will always see only one shadow from each object.) I moved
the flashlight around to different places, asking them to predict what
would happen to the shadow (where would it move? would it get taller
or shorter?) each time. This sounds very simple, but it took a few
iterations before they got it. I think this topic was right at their
level.
Next, I turned off the flashlight and put a yellow plastic film in
front of the projector. We saw that the shadow was still black,
because no light was getting there. Next, I turned on the white-light
flashlight and we saw that one shadow was yellow and the other was
white! That's because one shadow resulted from the statue blocking
the white light, and the other resulted from the statue blocking the
yellow light. Then they were eager to put another color plastic film
over the flashlight, say red to start with. I was careful to do this
in stages. I didn't mix the red flashlight light with the yellow
projector light until they had made a prediction. Then, I pointed the
flashlight somewhat away from the statue so that we saw how the light
mixed without thinking about the complication of the shadow. Then we
did the two-color shadows. Then we had a blast trying other color
combinations.
In each group, one person asked if we could put the red plastic and
the yellow plastic in front of the projector. First we discussed how
the plastic blocks all the colors in white light and only lets the
yellow through, so if I then put a piece of plastic which blocks all
colors other than red I should get what? Few of them got it, that it
would block almost all the light (the little light that did get
through appeared orange; exactly which shade depends on the details of
exactly how efficiently each film blocks each wavelength).
But that's not all! As more of a gee-whiz demo rather than teaching
principles of physical science, I had brought glow-in-the-dark paper.
I was not able to get the room very dark, so it was
unimpressive...until I whipped out a purple laser and started writing
on it with light! The laser is powerful enough that the spot where it
hits the paper really glows, and the bright glow lasts for several
seconds, so I could write a child's name and finish just as the first
letter was vanishing. This was a nice connection to other parts of
their curriculum: they were studying the deep sea and had just learned
about bioluminescence, and also they are learning their letters so
many of them could recognize their own names, and were extremely
gratified to recognize it when I wrote it.
For the groups which had a few extra minutes, I let them play with
either the slide projector and color films, or colored Christmas tree
lights, which make it easy to mix colors (warning: with the newer LED
lights the mixing is not that impressive because of the
specific-wavelength vs range-of-wavelengths issue discussed above).
In the last group, two kids stayed an extra 20 minutes while I packed
up and their peers were playing on the playground! So it seems to
have been interesting to them.
Diffraction gratings can be ordered from Edmund Scientific for about
$1 each, and I just ordered some for the school so kids can experiment
outside the very limited hours I am there. If you want to do more at
home with your child, I suggest getting a few strongish flashlights,
taping different color plastic films over them, and having fund after
the Sun sets.
so I decided to cover some of the same concepts again with a different
approach, as well as add some new concepts related to light. I bought
some powerful light sources from Big 5: an LED lantern and a big
LED flashlight, and I borrowed an old slide projector just to use as a
light source. (Note to self: check eBay for used slide projectors!)
I started by showing how white light is made up of many colors. I had
them look at a compact fluorescent bulb through a special piece of
plastic (a diffraction grating) which separates the different
wavelengths of light. Different wavelengths are perceived as
different colors, so they saw this:
 |
| The compact fluorescent bulb is at left. I stuck a diffraction grating in front of the camera lens, which separates the wavelengths (colors) of light so that we see a purple bulb, a blue bulb, a cyan bulb, a green bulb, and orange bulb, and a red bulb off to the right. This shows that what we perceive as white light is actually composed of a mixture of colors. |
They described it as a rainbow, and yes, it does show that white light
is composed of colors, but it's actually even more interesting than
that. (This part in parentheses I did not go over with the kids, but
it might be useful for other audiences. If the light bulb emitted a
continuous range of wavelengths, the colors would be spread out
continuously and would just be a smear like an actual rainbow is.
This is what an incandescent bulb would look like through a diffraction grating.
But the CF bulb emits only a few specific wavelengths, so you see a
well-defined image of the bulb in one very specific shade of purple,
another well-defined image in a very specific shade of blue, etc.
This is related to how the CF bulb gets its greater energy efficiency.
The incandescent bulb emits a very broad range of wavelengths, from
the ultraviolet to the infrared, and many of these are wasted because
human eyes cannot detect them. The CF bulb by design emits only a few
specific wavelengths which are all chosen to be visible, so none of
its output is wasted. Therefore, for a given amount of visible light
output (listed in lumens on the package), it uses much less energy
(listed as watts on the package). So always buy light bulbs by
looking at the lumens, not watts! You may also see CF packages which
say "100 W equivalent", which means they give off as many lumens as a
100 W incandescent bulb, even while they drain fewer watts from the
grid. And this is also why the light from a CF bulb is sometimes
ugly: engineers have to work hard to build a set of specific
wavelengths which convincingly fake a full range of wavelengths.)
We did talk about what colors might exist beyond violet on one end of
the spectrum and beyond red on the other end. I asked, and none of
them knew that ultraviolet light from the Sun can cause sunburn, so
now they know why their parents slather on sunblock so often! I also
mentioned that next time I will bring an infrared camera so they can
see what the world looks like in that "color."
Having established that white light is composed of many colors, I
asked them about their experience mixing paints of different colors.
What do you get when you mix many colors of paint together? Some of
them knew that you get a yucky dark mess, not white. So mixing colors
is different with light than with paint! (Again, this is above their
age range, but possibly useful to readers: the difference stems from
the fact that red paint, for example, is red because it absorbs all
colors other than red, and reflects red. So if you mix that with
paint which absorbs all colors other than green and paint which
absorbs all colors other than blue, you basically have paint which
absorbs all colors! Paint basically subtracts light rather than adds
light.)
Next, I fired up the slide projector, which I has set up with a small
statute near the screen so that the statue's shadow was clearly
visible. (In practice, I had determined that using a 5-year-old as
the shadowmaker was not practical; it was difficult to keep him still
and to give directions about moving right vs left, etc. I also
darkened the room as much as I could beforehand.) I then turned on a
powerful flashlight and showed how you get two shadows with two light
sources. (This is a powerful argument against Moon-landing-hoax
conspiracy theorists, by the way, who claim that the lighting in the
astronaut's videos came from multiple studio lights; look at the video
and you will always see only one shadow from each object.) I moved
the flashlight around to different places, asking them to predict what
would happen to the shadow (where would it move? would it get taller
or shorter?) each time. This sounds very simple, but it took a few
iterations before they got it. I think this topic was right at their
level.
Next, I turned off the flashlight and put a yellow plastic film in
front of the projector. We saw that the shadow was still black,
because no light was getting there. Next, I turned on the white-light
flashlight and we saw that one shadow was yellow and the other was
white! That's because one shadow resulted from the statue blocking
the white light, and the other resulted from the statue blocking the
yellow light. Then they were eager to put another color plastic film
over the flashlight, say red to start with. I was careful to do this
in stages. I didn't mix the red flashlight light with the yellow
projector light until they had made a prediction. Then, I pointed the
flashlight somewhat away from the statue so that we saw how the light
mixed without thinking about the complication of the shadow. Then we
did the two-color shadows. Then we had a blast trying other color
combinations.
In each group, one person asked if we could put the red plastic and
the yellow plastic in front of the projector. First we discussed how
the plastic blocks all the colors in white light and only lets the
yellow through, so if I then put a piece of plastic which blocks all
colors other than red I should get what? Few of them got it, that it
would block almost all the light (the little light that did get
through appeared orange; exactly which shade depends on the details of
exactly how efficiently each film blocks each wavelength).
But that's not all! As more of a gee-whiz demo rather than teaching
principles of physical science, I had brought glow-in-the-dark paper.
I was not able to get the room very dark, so it was
unimpressive...until I whipped out a purple laser and started writing
on it with light! The laser is powerful enough that the spot where it
hits the paper really glows, and the bright glow lasts for several
seconds, so I could write a child's name and finish just as the first
letter was vanishing. This was a nice connection to other parts of
their curriculum: they were studying the deep sea and had just learned
about bioluminescence, and also they are learning their letters so
many of them could recognize their own names, and were extremely
gratified to recognize it when I wrote it.
For the groups which had a few extra minutes, I let them play with
either the slide projector and color films, or colored Christmas tree
lights, which make it easy to mix colors (warning: with the newer LED
lights the mixing is not that impressive because of the
specific-wavelength vs range-of-wavelengths issue discussed above).
In the last group, two kids stayed an extra 20 minutes while I packed
up and their peers were playing on the playground! So it seems to
have been interesting to them.
Diffraction gratings can be ordered from Edmund Scientific for about
$1 each, and I just ordered some for the school so kids can experiment
outside the very limited hours I am there. If you want to do more at
home with your child, I suggest getting a few strongish flashlights,
taping different color plastic films over them, and having fund after
the Sun sets.
Saturday, December 10, 2011
Let there be light!
Back to Primaria (pre-K/K) this week. The teachers asked me to
explain how lenses work, because the kids had been making toy
eyeglasses out of pipecleaners and were curious about it. I had long
wanted to do some demos with light anyway. It takes a lot of trouble
to make a room really dark (so that the light relevant to the
demonstration is more visible) during school hours, so I figured I
would go to that trouble and combine topics. Linus (my son in
Primaria) had asked just a week or so before about the Moon. He
thought the phases of the Moon were due to Earth's shadow falling on
the Moon. I pointed out that the crescent Moon appears not too far
from the Sun, so the Earth's shadow cannot be falling on it. He came
up with some crazy stuff about light bouncing back and forth, back and
forth between Earth, Sun, and Moon "like an air hockey puck." So I
had a motivation to do phases of the Moon with the kids, but first I
had to build on basic concepts of light, like the difference between
emission and reflection (the Sun emits light and is the source of
light in our solar system; the Moon reflects some fraction of the
light it receives, but not enough to illuminate the other bodies in
the solar system).
So I set up in the kids' bathroom, which is the only room with no
windows. I still had to spend a lot of time taping up the open
doorway with black plastic to prevent a lot of light coming in. In
groups of 5-6, the kids came in and we started by talking about how we
couldn't see anything without a source of light. I then turned on an
unexpected source of light: a laser pointer. We discussed how they
couldn't see the source of light directly, but they could see the
light when it reflected off the ceiling. Next, a flashlight. I
pointed it directly at them, then pointed it at the ceiling. So a
given light source can be seen directly or indirectly (reflected)
depending on your relationship to it.
Now I turned on the "Sun": a naked light bulb. Unlike a flashlight or
laser pointer, it emits in all directions. But can we always see the
Sun? We discussed various reasons for not seeing the Sun, such as
clouds. But when is it really dark? At night. And what is night?
"Clouds" were again offered as a reason, so we discussed what happens
just before night: "the Sun goes down behind the mountains." So then
we each pretended we were the Earth, and slowly turned around so that
the Sun came into and out of our field of view. [The next day, my
wife Vera offered a really good suggestion: have them extend their
arms to make a "horizon" which turns with them.] To be honest, a lot
of kids spun way too rapidly and weren't really getting it. I
repeated the whole thing with a globe. We agreed on the location of
California and looked at how California varied between bright and dark
as the Earth turned. A problem with this is that light reflecting off
the walls provides a non-negligible amount of illumination for the
back side of the Earth, and the effect is not nearly as dramatic as
you would thing. Vera suggests decoupling the day/night concept from
the light demo, just pasting up a picture of the Sun in a regular
classroom and doing the horizon thing. I think she's right about
that! Another possibility is to build a little model. If the Sun were
a Christmas-tree bulb and the Earth a nearby marble, relatively little
light would bounce off the walls of the room!
Next, we tackled phases of the Moon. I had one child volunteer to be
Earth while I took a volleyball Moon and moved it around Earth,
showing how the Earth-person sees a fully-illuminated Moon when it is
opposite the Sun, and sees (rather, does not see) an un-illuminated
Moon when it is more or less between Earth and Sun. However, this did
not work well for several reasons. Each group had a bunch of other
kids who were not the Earth and saw the whole thing from a variety of
vantage points. It was very difficult to steer the kids into seeing
what they were "supposed" to see. One girl said "now I'm the Earth"
when the Moon happened to come close to her. In one group, the
Earth-volunteer gave the wrong answer when I asked him whether the
side of the Moon he was seeing was bright or dark; I think he just
didn't know what to compare to, so I need to be more careful about
exactly how I word my questions.
Finally, the lens. The key to a good visualization is to avoid using
all three dimensions. I put a flashlight on a table so they can see
how the light spreads out by looking at the light and dark patterns on
the table. I put a comb in front of the light to give a visual
impression of light rays spreading out on the table. Then I set a
special lens on the table, which is like a slice of a lens so that it
can sit flat on the table. This shows that the light rays which go
through the lens are bent so that they converge back together rather
than continue diverging. It's quite striking if set up right. I had
a card which I pretended was a movie screen, and projected the focused
image there. We talked about movie theaters and where they would sit,
did they ever look behind them and see the bright light coming out of
the lens, and what would happen if there was no screen. For the
groups which had a bit of time left at the end, I moved the lens
around to show that if it's too close to the light, it's not powerful
enough to converge the light. It might be powerful enough to stop
further spreading of the light, though, and I showed how a second lens
could then converge that light. The idea was to show that there are
many combinations and possibilities.
I felt that the kids were more disengaged than usual, and I felt that
it was directly attributable to the "demo" rather than "hands-on"
nature of the activity. I made the "demo" decision because I felt it
would be chaos to have 4- and 5-year-olds handling flashlights and
lenses in teams of two or three. That may have been correct, but I
should have found some way to prevent the whole 20 minutes from being
all demo. One possible structure is the sandwich: an initial demo
followed by hands-on activities with a more complicated demo or
summary discussion at the end. But this didn't fit with the list of
topics I wanted to cover. I realize now that I was too much in
"professor" mode: practicing inquiry is more important than covering a
list of topics! The kids brought up (indirectly) one thing I had
thought about last year but forgotten: setting up a light so that they
can make shadows themselves. They love doing this, and if I set it up
right they can actually explore different aspects of light. For
example, I could set up two lights of different colors and they could
see how to control the color of the shadow. Or they could explore how
a given object can have differently shaped shadows depending on its
orientation to the light. I can even imagine setting up a "light
studio" which they could play with during the week before or after my
visit.
explain how lenses work, because the kids had been making toy
eyeglasses out of pipecleaners and were curious about it. I had long
wanted to do some demos with light anyway. It takes a lot of trouble
to make a room really dark (so that the light relevant to the
demonstration is more visible) during school hours, so I figured I
would go to that trouble and combine topics. Linus (my son in
Primaria) had asked just a week or so before about the Moon. He
thought the phases of the Moon were due to Earth's shadow falling on
the Moon. I pointed out that the crescent Moon appears not too far
from the Sun, so the Earth's shadow cannot be falling on it. He came
up with some crazy stuff about light bouncing back and forth, back and
forth between Earth, Sun, and Moon "like an air hockey puck." So I
had a motivation to do phases of the Moon with the kids, but first I
had to build on basic concepts of light, like the difference between
emission and reflection (the Sun emits light and is the source of
light in our solar system; the Moon reflects some fraction of the
light it receives, but not enough to illuminate the other bodies in
the solar system).
So I set up in the kids' bathroom, which is the only room with no
windows. I still had to spend a lot of time taping up the open
doorway with black plastic to prevent a lot of light coming in. In
groups of 5-6, the kids came in and we started by talking about how we
couldn't see anything without a source of light. I then turned on an
unexpected source of light: a laser pointer. We discussed how they
couldn't see the source of light directly, but they could see the
light when it reflected off the ceiling. Next, a flashlight. I
pointed it directly at them, then pointed it at the ceiling. So a
given light source can be seen directly or indirectly (reflected)
depending on your relationship to it.
Now I turned on the "Sun": a naked light bulb. Unlike a flashlight or
laser pointer, it emits in all directions. But can we always see the
Sun? We discussed various reasons for not seeing the Sun, such as
clouds. But when is it really dark? At night. And what is night?
"Clouds" were again offered as a reason, so we discussed what happens
just before night: "the Sun goes down behind the mountains." So then
we each pretended we were the Earth, and slowly turned around so that
the Sun came into and out of our field of view. [The next day, my
wife Vera offered a really good suggestion: have them extend their
arms to make a "horizon" which turns with them.] To be honest, a lot
of kids spun way too rapidly and weren't really getting it. I
repeated the whole thing with a globe. We agreed on the location of
California and looked at how California varied between bright and dark
as the Earth turned. A problem with this is that light reflecting off
the walls provides a non-negligible amount of illumination for the
back side of the Earth, and the effect is not nearly as dramatic as
you would thing. Vera suggests decoupling the day/night concept from
the light demo, just pasting up a picture of the Sun in a regular
classroom and doing the horizon thing. I think she's right about
that! Another possibility is to build a little model. If the Sun were
a Christmas-tree bulb and the Earth a nearby marble, relatively little
light would bounce off the walls of the room!
Next, we tackled phases of the Moon. I had one child volunteer to be
Earth while I took a volleyball Moon and moved it around Earth,
showing how the Earth-person sees a fully-illuminated Moon when it is
opposite the Sun, and sees (rather, does not see) an un-illuminated
Moon when it is more or less between Earth and Sun. However, this did
not work well for several reasons. Each group had a bunch of other
kids who were not the Earth and saw the whole thing from a variety of
vantage points. It was very difficult to steer the kids into seeing
what they were "supposed" to see. One girl said "now I'm the Earth"
when the Moon happened to come close to her. In one group, the
Earth-volunteer gave the wrong answer when I asked him whether the
side of the Moon he was seeing was bright or dark; I think he just
didn't know what to compare to, so I need to be more careful about
exactly how I word my questions.
Finally, the lens. The key to a good visualization is to avoid using
all three dimensions. I put a flashlight on a table so they can see
how the light spreads out by looking at the light and dark patterns on
the table. I put a comb in front of the light to give a visual
impression of light rays spreading out on the table. Then I set a
special lens on the table, which is like a slice of a lens so that it
can sit flat on the table. This shows that the light rays which go
through the lens are bent so that they converge back together rather
than continue diverging. It's quite striking if set up right. I had
a card which I pretended was a movie screen, and projected the focused
image there. We talked about movie theaters and where they would sit,
did they ever look behind them and see the bright light coming out of
the lens, and what would happen if there was no screen. For the
groups which had a bit of time left at the end, I moved the lens
around to show that if it's too close to the light, it's not powerful
enough to converge the light. It might be powerful enough to stop
further spreading of the light, though, and I showed how a second lens
could then converge that light. The idea was to show that there are
many combinations and possibilities.
I felt that the kids were more disengaged than usual, and I felt that
it was directly attributable to the "demo" rather than "hands-on"
nature of the activity. I made the "demo" decision because I felt it
would be chaos to have 4- and 5-year-olds handling flashlights and
lenses in teams of two or three. That may have been correct, but I
should have found some way to prevent the whole 20 minutes from being
all demo. One possible structure is the sandwich: an initial demo
followed by hands-on activities with a more complicated demo or
summary discussion at the end. But this didn't fit with the list of
topics I wanted to cover. I realize now that I was too much in
"professor" mode: practicing inquiry is more important than covering a
list of topics! The kids brought up (indirectly) one thing I had
thought about last year but forgotten: setting up a light so that they
can make shadows themselves. They love doing this, and if I set it up
right they can actually explore different aspects of light. For
example, I could set up two lights of different colors and they could
see how to control the color of the shadow. Or they could explore how
a given object can have differently shaped shadows depending on its
orientation to the light. I can even imagine setting up a "light
studio" which they could play with during the week before or after my
visit.
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