In my 41 years of teaching high school students in South Bend,
Indiana, one of my favorite units of discussion was the unit on
light. The nature of light can challenge the best minds of high
school physics classes. It is easy to show experimentally the nature
of light making it an ideal subject to get kids to think outside the
box.
I always began the discussion of the nature of light by doing
experiments with sound and mechanical waves. You can show not only
such things as frequency and wavelength, but it is easy to show that
there are two kinds of waves and that sound and light waves are very
different. Put a doorbell in a vacuum jar and suck all the air out
and the sound diminishes until you cannot hear it at all — and yet 
you can see it. Obviously sound waves cannot travel
through a vacuum and light can. You can then use a ripple tank and
get kids to see that mechanical waves do things you can
watch — diffract around an obstacle, refract when the density of the
material changes, reflect, and interfere with each other. Sound can
easily be shown to do all of these things, and so can light.
The fun stuff really starts when you show students that light has
properties of mass — it can knock electrons out of crystals in
something called the photoelectric effect (seen in solar cells). I
would yell at the same crystal and show that sound could not do
that. We would then show that while light has momentum and thus
properties of mass, when light stops it has no mass. Shining a light
on someone does not cause them to gain weight. The final teaser was
to show that light can be polarized — made to vibrate in one plane.
We would make our own 3-D pictures using the simple polarizers we
had in the lab, or show how polarized sunglasses work. We would then
show that light is two dimensional — it has an X and a Y component
but no Z component. We would also see that light comes in all kinds
of frequencies just as sound does, and that anything you can do with
visible light you can do with radio waves, X-rays, microwaves, or
infrared radiation.
The question of how light can be a wave and a particle has perplexed
the minds of scientists for centuries. The fact that it has these
properties allows us to have all of the technology and modern
conveniences that we enjoy today. It is also, of course, what gives
us the beautiful sensation of color and sight. I had one student
many years ago who was trying to grasp all of this and apply it to a
problem-solving exercise the students were working on. He looked up
at me in bewilderment and said “Who thought all of this stuff up?”
He then brightened and smiled and said, “Never mind, I think I just
answered my own question.” The simple phrase “Let there be light” is
an incredibly complex command.
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