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.
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