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.