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.

100 Days

Primaria has been counting the days of school since it started in
September, to give the kids practice counting.  Last week they got up
to 100, which was a big milestone they celebrated.  So I sneaked in an
extra science activity when they did PE.  The Sun is about 100 times
bigger than the Earth, which I thought made a nice connection between
science and what they were celebrating.  I brought a 12-inch (1-foot)
globe to represent the Earth, and marked out a 100-foot diameter
(actually, 109 feet, to be more accurate) circle in the park to
represent the Sun.  I marked it out with about a dozen cones, and to
help visualize a real circle I draped a 50-foot rope around the edges
of the first 2-3 cones; more would have been better.

It's one thing to say the the Sun is 109 times bigger than the Earth,
but another to see it!  If you are familiar with the park, imagine the
Sun covering all the grass in the narrower east-west direction, and
most of the grass in the north-south direction.  Compare that to
little old Earth, the 12-inch globe in my hand!  This is the beginning
of a scale model, but to be true to the scale model we would have to
put Earth a few miles away, like in downtown Davis!  The Sun looks
small only because it is really far away.

Part of the reason that saying "the Sun is 109 times bigger than the
Earth" has less impact than it should is that we are comparing
diameters, not volumes.  The large area of grass I marked is like a
cross-section of the Sun, 109 times Earth's size in one dimension AND
109 times Earth's size in the second dimension as well, for over
10,000 times the area.  But we should really try to visualize a sphere
109 feet tall as well, which would make for a volume over a million
times the volume of the globe in my hand.  So we could just as easily
say that the Sun is over a million times Earth's size, if we are
talking about volume.  See my post(s) on scaling relations
for more on this.  I didn't try to get the Primaria kids to think
abstractly about this, but I did ask them to imagine the circle
representing the Sun as extending 100 feet high.  Then we ran around
the Sun a few times.

Under Pressure

The Primaria kids are learning about the ocean, starting from the deep
sea and moving up, so I decided to focus on pressure Friday.  I
started with a very simple giant syringe with the end capped, mounted
in a wooden block for stability.  I filled it with water and had each
of them press down as hard as they could.  Many of them already knew
(having encountered it when they studied the Marianas Trench) that
pressure at any point in the ocean is just the result of the weight of
the water above that point pressing down.  The syringe reminded them
of this point and made it vivid; they were playing the role of the
upper layers of water pressing down, and we imagined how much pressure
a creature would feel if it were in the syringe when they pressed on
it as hard as they could.  (Warning to future self: supervise more
closely because the syringe is easily broken!)

As a bit of an aside from the main focus of water pressure, we then
filled the syringe with air and repeated a round of pressing.  What's
different is that the air shrinks in response to the pressure!  The
plunger actually goes down when you press on it, and you can see that
that is not due to escaping air because the plunger springs right back
when you let go (assuming you have a good syringe where air really
does not leak around the plunger).  It's an interesting feeling to
feel the air pushing back like that; it really feels like squeezing an
invisible spring.  Water is very different: it pushes back without
changing its volume.  Physicists would say that air is compressible
and water is incompressible.

Next, we did the "three-hole can" experiment.  This is just a vertical
glass tube with three stoppered holes at different heights.  Fill with
water, and ask the kids to predict what will happen.  After they say
water will squirt out the holes, ask them if it will squirt out
equally fast (or far) from each hole.  You will probably get a variety
of opinions, at which point you can talk about the importance of doing
experiments to settle issues in science.  If there is a unanimous
prediction, I ask if we should still do the experiment, and we
conclude yes, because sometimes everybody is wrong.  This worked out
well Friday because one group had a unanimous prediction which was
correct, and then later had a unanimous prediction which was
incorrect, so they experienced both sides of it. 

In any case, if you unstopper all three, it's clear that water squirts
out fastest from the lowest hole.  This is because it is under more
pressure, having more weight of water above it.  (Technical note: if
you judge the pressure by how FAR the water travels before hitting the
ground, the lowest hole is at a bit of a disadvantage because its
water has less time before it hits the ground.  But that's a minor
factor for most setups.)

Next, I had set up a big tub of water with a stoppered hole at the
same height as one of the holes in the tube.  I restoppered the tube
when the water levels in the two containers matched, so the heights of
the water, as well as the heights of the hole, match.  Now the kids
have to decide, having removed the variable of height, which will
squirt out faster when unstoppered: the skinny tube or the massive
reservoir of water.  They turn out to be equal; pressure is determined
only by the height of the water above you, not by the volume.  A
practical application of this is that the strength required of a dam
is determined by the depth of the water it holds back, not by the
volume of the lake.

Next, we looked at how you can hold water in a straw by putting your
finger over the top.  This is related to pressure because if the water
started to fall out of the straw without air getting in, the air in
the straw would have to occupy more volume and thus be at lower
pressure.  The higher pressure of the outside air then provides an
upward push on the water to prevent it from falling.  (I simplified
this a bit for the kids; the summary for them was that the water could
not fall out without a way for air to get in to fill the space it
left.)  Then came the cool part; I have a similar setup in which air
is prevented from escaping from an inflated balloon, and it is amazing
to see how the balloon stays inflated even when you let go of the
neck!

Here's how it works. It start with a piece of glass which is shaped
more or less like an inflated balloon.  The exact shape doesn't
matter, but it needs a good amount of space inside for the balloon to
inflate, and it also needs a neck on which to mount the balloon's
neck.  Stuff most of the limp balloon inside, and mount the balloon's
neck on the glass container's neck.  Then inflate the balloon, which
surreptitiously pushes air out of the glass container through a hole
in its back side.  Then insert a stopper into that hole, and there's
no way the balloon can deflate; in order to do so, you would need to
remove the stopper so air can take up the space in the glass
container.  So take your mouth off the balloon's neck and watch the
kids gape...its neck is held wide open by the glass neck, but the
balloon does not deflate!

You can then ask them to predict what will happen when you remove the
cork, and there are further variations such as pouring water in the
balloon before removing the stopper (which creates a nice squirt of
water when you do remove the stopper). 

I found that the whole thing went pretty quickly, and there would have
been time to also do the vortex bottle, which I will write about next
time.  Some groups had extra time to blow up balloons and release
them, and do other hands-on experiments with the equipment, but that's
always a good thing.

I also (re)discovered something about managing the kids.  I had them
all sit around a big round table, and that eliminated a lot of the
annoyances which had wasted time in previous activities: kids
jockeying for better position, kids getting distracted by the
playground equipment (we were outdoors for obvious reasons), etc.  In
the future I should set things up so that they are seated if at all
possible.

A simple, fun extension of this activity you might want to do at home is to
make a giant version of the three-hole can.  Stand an 8-foot PVC pipe
straight up, make the bottom end watertight, drill a bunch of holes in it,
and put a hose in the top.  This will make a good thought-provoking
sprinkler for the summer!

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:

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.)

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.