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Water Prism

This week's experiment is dedicated to my mother. Not only because of all the things she has done for me, but also because she gave me the idea. When I called her this morning to wish her a happy Mother's Day, she mentioned that the sunlight coming through the window was bouncing off the mirror and making a rainbow on the floor. That started me on a quest for rainbows, and I spent quite a bit of time playing with them. To experiment with rainbows, you will need:

- a bowl of water
- a mirror small enough to fit in the bowl
- a darkened room
- a beam of sunlight or a strong flashlight

If you are using sunlight, you need to find a room where you can close all the shades. Open one curtain just a bit, to let in a small beam of light. Place the bowl of water on a flat, stable surface, so that the beam of light is shining on the water. Place the mirror into the water, at an angle. Lean it against the side of the bowl. You want the light to enter the water, reflect off the mirror and shine on the wall or ceiling.

As the water ripples, you will see squiggles of light dancing around. I spent quite a bit of time just playing with that. I even laid one of the stereo speakers on its side and placed the bowl of water on top. This made the water vibrate with the music, which meant that the patterns of the light also followed the music. It worked better with some songs than others, so if you try it, try several songs.

Now, what about the rainbow? Depending on the angle of your mirror and the angle of the light, you may already see it. If not, then move the top of the mirror slowly towards the center of the bowl. As the spot of light moves downward, you should start to see color at its edges. One edge will be blue, and the other will be red. Tilt the mirror more and you will see more colors. Keep adjusting it until you get a nice rainbow.

Why does this make a rainbow? When light passes from a substance of one density to a substance with a different density, it bends. So light bends when it moves from air to water, from water to air, from air to glass, from water to glass, etc. This bending is called refraction, and you can see it easily by sticking your finger into a clear glass of water while looking from the side. The refraction will make it look as if your finger were broken, with the part that is under water moved to one side.

Usually, if you want to see the spectrum of light, you use a triangular piece of glass called a prism. Light enters through one side of the prism, bending as it enters. Each color bends at a different angle, but the difference is so slight that you usually don't see it. Then the light moves through the glass to the other side. Because the light hits this side at an angle, the colors are spread farther apart, just as a flashlight beam stretches out when you shine it on the floor at a shallow angle. As the light moves from the glass to the air, it is refracted again, and again the different colors bend at different angle, spreading the colors even more.

Each of these steps moves the different colors farther apart, so when the light shines on the wall, you see the colors of the rainbow. But, what does that have to do with a mirror in a bowl of water? Look at the bowl from the side. The surface of the water and the mirror form an angle, just like the sides of a prism. By adjusting the angle of the mirror, you are adjusting the angle for the sides of your prism of water. When you get the angle right, the colors are spread enough for you to see the rainbow colors.

You can experiment with different bowls and different water levels to give you different angles. You will find that some give you better colors than others. You might also try adding some food coloring to the water. What do you think that would do? Or what would happen if you used clear water, but let the light pass through a colored piece of plastic first, so that you had a beam of colored light entering the water? Would you still see a rainbow?

Now for the tough question. What would happen if you had two prisms? Would the second prism recombine the colors into white light? No. Although many textbooks show this, it does not happen. It is one of those misconceptions in science, like vultures circling over dying animals, lightning hitting Ben Franklin's kite, and the Coriolis effect causing the swirl of water when you flush the toilet. All are wrong, though they may pop up in your textbook. I happened to look at my niece's science book recently and was amazed to find the two prism mistake, along with number of other error. OK, OK, I will get off my soapbox and go have a bowl of ice cream.

Have a wonder filled week.