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.
No comments:
Post a Comment