Easy Rainbow Experiment #428
Grade Level(s):

This week's experiment came as a result of housecleaning. Being here, I have finally had some time to get the travel trailer organized. As I was moving things around, I noticed the most beautiful rainbow on the ceiling.

Materials:

To see for yourself, you will need:

a CD
a flat surface in direct sunlight
a lamp
foil
a hair
a feather

Plan:

Place the CD on a flat surface, in direct sunlight. You want the label side down, so that the sunlight will reflect off of the side that holds the information. Then look at the ceiling. You should see a wonderful rainbow. Why?

We normally think of light as traveling in straight lines, but on a very small scale, things are different. When a beam of light hits the edge of an object, it is bent slightly. This bending is called diffraction. That is a word that most people are not familiar with, so lets take a closer look.

Turn on a nice, bright lamp. Take a piece of foil between you and the lamp. I used a lamp with a shade, and placed the foil across the top of the shade. Make a tiny hole in the foil, to let a pinpoint of light come through. That will be our light source.

Next you need a hair. Since I am "hair challenged", I got one from Mawra the Cat's brush. Hold the hair between your eye and the pinpoint of light. The hair should be 2 or 3 inches from your eye to start. You may have to move it around a bit. When you get it right, you will see a tiny line of light crossing the hair. You won't actually see the hair, because it will be out of focus, but you will see the band of light at a right angle to the hair. If the hair is vertical, the band will go from side to side. If the hair is horizontal, then the band will go up and down. If you look closely, you may see that the band is actually made up of tiny dots of light. That band is caused by light bending around the edge of the hair. As the light waves bend, they interfere with other light waves, producing the light pattern you see.

What would happen if you had more hairs, all arranged side by side? That is where our feather comes in. The feather is made up of many hair-like structures, all lined up side by side. Hold the feather a couple of inches from your eye and look at your pinpoint of light through the feather. What do you see?

Rainbows! You may have to move the feather around a bit, and try different parts, but you should be able to find a spot where you see rainbow colors. All the lines arranged side by side form what is known as a diffraction grating. It bends the light to separate the colors.

Look closely at the CD and you should see that it also has many tiny lines, side by side. These lines do the same thing that the feather did, only much better, spreading the light to make the rainbow on your ceiling. Diffraction gratings are very useful things. Scientists use them to examine the light from stars to help find out what they are made of, their temperature, and many other things. Chemists use them to identify chemicals. They are even used to make the rainbow glasses that you buy in museum gift shops.

Well, Mawra the Cat has fallen asleep on my rainbow CD, so I will get back to my house cleaning.

From Robert Krampf's Science Education Company

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