Learn

The world is full of CD cases, although iPod-like devices that store music on hard and flash drives are slowly making the CD go the way of the LP, and 8 track and cassette. But before they go - here is a nice way to re-use your old CD cases and learn something about light diffraction too.

• A CD (compact disc)
• Sunshine or a flashlight
• A room that is dark and
• A piece of white paper (or a white wall)

1. Take the CD and look at the blank side (the side that doesn’t have any printing on it). See bands of color? If you tilt the CD back and forth the colors will shift and change.
2. Hold the CD in the sunshine (or use your flashlight). Put the piece of white paper so that the light reflecting off the CD shines onto the paper. The reflected light will make fabulous rainbow colors on your paper.
3. Tip the CD and see how that changes the reflections. Change the distance from the CD to the paper.
4. Look closely at the CD. It’s made of aluminum coated with plastic. The colors that you see on the CD are created by white light reflecting from ridges in the metal.

Why does a CD reflect rainbow colors?

Like water droplets, the CD separates white light into all the colors that make it up. The colors you see reflecting from a CD are interference colors, like the shifting colors you see on a soap bubble or an oil slick.

You can think of light as being made up of waves, like the waves in the ocean. When light waves reflect off the ridges on your CD, they overlap and interfere with each other. Sometimes the waves add together, making certain colors brighter, and sometimes they cancel each other, taking certain colors away.