Comparing Spectra of Different Light Sources

The underside of a CD or DVD abounds with magical colors. You can study the spectra of different light sources very easily with a blank CD or DVD and some light bulbs.

Materials

• Blank CD or DVD

• Colored photography filters or colored cellophane

• Different light sources: incandescent, fluorescent, full spectrum (daylight bulb), halogen, neon, and xenon bulbs

What To Do

1. Hold the underside of the CD near each of the light sources. This is best done at night to eliminate background light and isolate the light source.
2. What colors do you see for each light bulb?
3. Hold a colored filter or cellophane paper near your eye. What happens to the colors with a red filter? A blue filter? A green filter? A yellow filter?
4. Do the filters produce differences under an incandescent bulb, a fluorescent bulb, a full spectrum light, a halogen bulb, a neon bulb, and a xenon bulb?

How It Works

Light travels through space in waves with varying frequencies and wavelengths. Any object (like a prism) that bends the light waves separates the light into the colors of white light: red, orange, yellow, green, blue, indigo, and violet. This is called a spectrum. The CD acts like a prism because the narrow grooves are so close to each other that you cannot see the separation. But, white light will be separated by the grooves.

Another name for this CD phenomenon is diffraction. See the photographs of spectra caused by diffraction gratings.

The size or width of the spectrum you see is dependent on the size of the light bulb. The smaller the light bulb, the smaller or narrower the spectrum. The greater the angle between the CD and the light, the more iridescent the color. This is true for the full spectrum and halogen bulbs. The fluorescent bulb gives off white light from the phosphors on the inside of the glass. The spectrum is not as pronounced as with the others. The colored filters may absorb the colors which are not the filter color.

Variations

Put the CD in a bowl of water and put the bowl under the light bulbs. You will still see the spectra, but it will be at a steeper angle because light waves are refracted or bent as they pass through a medium other than air.