A CD shows rainbow bands because its closely spaced spiral tracks act as a diffraction grating. When white light reflects from the disc, different wavelengths reinforce one another in different directions. Your eye sees separated bands of color as you tilt the disc or move the light.
What do the colorful rings in a CD photo show?
A compact disc has a reflective layer and a closely spaced microscopic spiral track. The spacing is tiny compared with what your eyes can resolve, but it is just the right scale for visible light to produce interference patterns. In the image below, the bright rings are a light-and-angle effect on the disc, not colored pigment painted onto it.
Why do the colors separate instead of reflecting as white?
White light contains many visible wavelengths. Waves reflected from adjacent parts of the closely spaced track travel slightly different distances. For some viewing angles, waves of one color add together strongly; at a different angle, another color becomes bright. This wavelength-dependent interference produces a rainbow.
A glass prism also separates colors, but mainly because different wavelengths refract by different amounts as they pass through glass. A CD’s rainbow is primarily caused by diffraction and interference at its repeated microscopic spacing. Both can produce spectra, even though the mechanisms differ.
Try a safe five-minute CD rainbow experiment
Take an old, clean CD and a small white-light flashlight into a room with dim lighting. Point the flashlight at the shiny side and slowly tilt the disc, looking at the light reflected away from the flashlight beam. Change your angle until the rainbow bands appear; then move the flashlight and watch the pattern shift. Never use a laser pointer for this experiment, and do not direct strong reflected light into anyone’s eyes.
Does a scratched CD make fewer rainbows?
Scratches, fingerprints and dust can scatter light and disrupt crisp patterns in places. However, a disc with minor scratches can still produce colorful bands because the underlying repeating track structure remains. Even a disc that no longer plays properly may still show diffraction.
Frequently asked questions
Is the rainbow connected to the songs on the CD?
Only indirectly. The physical track structure that stores information is also what creates the optical pattern. The music itself does not determine the rainbow colors.
Do DVDs make rainbow colors too?
Yes. DVDs have an even more closely spaced track structure, and their diffraction patterns can look different from those of CDs.
Why does the rainbow move when I tilt the CD?
The angles that reinforce each wavelength change relative to your eye and the light source, so the bright colors shift.
For the related technology, read how a CD player reads digital music. For a different source of rainbow colors, see why oil films look rainbow-colored.
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