View a spectrum in a CD

Look at a light source reflected in a CD and you can see its spectrum.

A small bright light source such as a minimag light makes a nice linear spectrum reflected in a CD


Small light sources, or distant light sources. A minimaglight with the reflector removed is a great light source.

Never look at the sun even reflected in a CD.

A compact disk. (All music CD's work, computer CD's should be recorded until they are nearly full.)

To Do and Notice

Use the CD as a mirror and find the reflected image of a small light in the CD.

Rotate the CD or move your head until you see a spectrum of light.

What's Going On?

The CD has closely spaced spiral tracks of pits which record digital information. The tracks are separated by 1.6 micrometers, 1.6 x 10-6 m.

These tracks act as grooves which diffract light

Each color of light is diffracted through a different angle. Thus a white light source which emitting a wide range of colors is spread into a spectrum by the CD.

To make a good spectrum the light source should have a small angular size, thus it should be small and close, but it could be large and distant as well.

Scientific Explorations with Paul Doherty

© 2004

15 May 2004