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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 Sunyou 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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