CDspectrometer

CD spectra

Introduction

Use a compact disk to separate light into its spectrum.

Material

A compact disk
a small light source such as a minimag light with the reflector removed, a distant lightbulb, or a Christmas tree bulb.
The source must be small!
Do not use the sun

Assembly

Turn on a bright small source of light.

To Do and Notice

Look into the clean side of a compact disc, CD, the side without a label.

Look at the light from a small source reflected off a compact disk, tilt the disk until you see a spectrum.

Use the CD like a mirror and tilt it until you can see the bright light reflected in the CD.

Once you see the reflected image of your light source experiment with tilting the compact disc until you can see a spectrum containing all the colors of the rainbow.

What's Going On?

The music on compact disks is digitally recorded.
Spiral tracks of ones and zeros are recorded on the mirror surface of the disk. These tracks are so close together, that they act as a diffraction grating for light. Adjacent tracks are separated by 1.6 microns, 1.6 x 10^-6 m. (Note DVD's have tracks spaced 0.74 x 10^-6 m apart and so spread the spectrum of light over a wider angle.

The light is spread into a spectrum perpendicular to the tracks.

Each color bends at a particular angle.
This is why you have to adjust the angle of tilt of the disk to bounce the spectrum into your eye.

To see the spectrum it must reflect and diffract off the disk into your eye.
Therefore, you you must move the compact disk around until you see the spectrum.

Going Further

Examine the spectra produced by other small light sources such as distant incandescent bulbs, distant sodium vapor street lights, and neon tubes.