Shadow Colors Science Experiment

This week's experiment came from my observations while setting up for a science show this week. I was at a school which actually had a stage (many places I perform in the cafeteria, library, etc.) and someone had turned on some colored lights as well as the normal stage lights. As I set up, I noticed some strange things about the shadows.

To see for yourself, you will need:

  • a dark room
  • a white light
  • a red light (other colors will work, but I found red the easiest to see)
  • a white wall or large sheet of white paper

You can buy a cheap, red light at most hardware stores, but you may already have several different colored lights in your decorations for Christmas or other holidays. Try to get a white light and a colored light that are about the same brightness. I wound up with a 40 watt red bulb and a 100 watt white bulb, but I still got it to work by using a lamp shade to dim the white one a bit.

First, shine the white light onto the white wall. Put your hand in between the light and the wall and look carefully at the shadow. Watch the shadows as you move your fingers around. See if you can make a shadow that looks like a bird, an alligator, or your favorite dinosaur. I managed to make one that looked exactly like my hand. The shadows look normal, dark areas on the white wall.

Turn off the white light and turn on the red one. Try making shadow pictures again. It works pretty much the same, except that now the wall looks red instead of white. The shadows are still dark. Now it gets interesting. With the two lights about a foot apart, turn on both lights. With both lights shining on the wall, it will probably look white. The white light tends to overpower the red. Place your hand to make a shadow again. It has two shadows now, one from each light. One shadow will be red. The other shadow Where did the green come from?

Well, the shadow is not really green. It looks green, but that is a result of the red light surrounding it. We have seen in past experiments that if you stare at a bright, red object, and then look away, you will see a green afterimage. The color sensitive cells in your eye get "tired" of seeing the red and when you look away, they react more to the other colors than to the red, leaving a green image.

The same thing is happening with the shadows. If you focus carefully on the green shadow and switch off the red light, you will see that it does not change color, but it no longer seems green. I know that may sound like a contradiction, but if you try it, you will see what I mean. Even knowing what was going on, I had to turn the lights on and off several times to convince myself.

You should try other colors as well. A green light will seem to produce shadows which are red. What color shadow do you think you would get from a yellow light? Purple! Really! Well, if you don't believe me, you will just have to try it yourself.

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