Reflecting Rainbows

 15 Minutes
What Do I Need?

  • compact disc (also known as a CD) (If you don't own any CDs, you can buy an old one at a garage sale. Or ask at a record store if they will give you a CD that won't play.)
  • sunshine (or a bright flashlight and a room that you can make dark)
  • piece of white paper
 Looking at the CD
 What Do I Do?
1Take the CD out of its case and take a look at the blank side (the side that doesn't have any printing on it). You'll see bands of shimmering color. Tilt the CD back and forth and the colors will shift and change.

Flashlight on the CD


2 Hold the CD in the sunshine. Or if it's a cloudy day, turn out the lights and shine your flashlight at the CD. Hold your 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.

Warning Message

 3 Tip the CD and see how that changes the reflections. Change the distance from the CD to the paper. What happens to the colors?

4 Take a close look at your 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.


When light reflects off or passes through something with many small ridges or scratches, you often get rainbow colors and interesting patterns. These are called interference patterns. Here are several other ways you can see interference patterns.

  • Squint at a distant bright light at night. You'll see starburst patterns around the light. If you look closely, you can see colors in the patterns. These patterns form when light bends around your eyelashes and imperfections in the layers that make up the lens of your eye. Tilt your head to one side while watching the pattern and notice that the pattern moves with you.
  • In a dark room, look at a bright light (maybe a candle flame) through a nylon stocking, a silk scarf, a feather, or a tea strainer. The pattern that you see depends on what you look through. Move the thing you're looking through and notice that the pattern moves with it.
  • Buy a set of "rainbow glasses" in a toy store or a science shop. Through these glasses, all lights look like rainbows. The glasses are made with diffraction gratings, clear plastic that is etched with many lines.

 

Why does a CD reflect rainbow colors?

Like water drops in falling rain, 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 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.


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This and dozens of other cool activities are included in the Exploratorium's Science Explorer books, available for purchase from our online store.

About the Books

Published by Owl Books,
Henry Holt & Company, New York,
1996 & 1997

ISBN 0-B050-4536 & ISBN 0-8050-4537-6,
$12.95 each






© 1998 Exploratorium