color_diffraction

The Spectrum of light revealed by a Compact disk

A compact disk can be used as a reflective diffraction grating

A compact disk mounted at 45 degrees to the output beam from an overhead projector.

Introduction

Use an overhead projector to project an image of a slit on a screen. A compact disk can be used as a diffraction grating to spread the white light passing through the slit into a spectrum from red through green to blue.

Material

An overhead projector and white screen.
A compact disk
Wide tape such as duct tape
A piece of cardboard large enough to mount the compact disk at a 45 degree angle in front of the overhead projector, plus some sticky tape.
Thin opaque cardboard, large enough to cover the overhead projector. I use manilla folders.
Transparent, colored pieces of plastic.

Assembly

Move the overhead projector close to the screen, perhaps 1 meter away.

Cover the top and bottom and middle of the compact disk with strips of tape as shown.

Compact disk and compact disk partially covered with tape.
The vertical lines represent the tracks on the compact disk.

Use the cardboard and tape to mount the taped-compact disk over the output beam from the overhead projector at a 45 degree angle. Keep the tracks on the compact disk vertical.

In the other piece of cardboard, cut a slit 4 to 6 mm (1/8 to 1/4 inch) wide and 10 cm (4 inches) long.

Place the cardboard with the slit in it on the overhead projector so that the slit is projected as a vertical line on the screen. (Note the compact disk will reflect the projected image to the side so, to project the slit on the front wall of the room, you will have to point the projector toward a side wall.)

Cover the entire overhead projector with cardboard so that no light gets out except the light that comes through the slit.

Focus the image of the slit.

Make sure the room lights are dim.

Notice the spectrum of light to one or both sides of the slit.

Wider slits will produce a brighter spectrum, narrower slits will produce a sharper yet dimmer spectrum. I suggest you start with slits about 6 mm (1/4 inch) wide.

You can move the position of the compact disk and the projector to optimize the brightness and size of the spectrum on the wall.

To Do and Notice

1. Notice the slit still appears white even after the compact disk is in-place, this is called the zero'th order diffraction.

Spectra of light appear to one or both sides of the slit, these are the first-order spectra.

In both first order spectra, blue light is near the white slit and red is farthest away.
Notice the colors that appear between blue and red.

2. Cover the top half of the slit with a transparent yet colored piece of plastic, e.g. yellow. The full spectrum of the white light from the bottom of the slit appears to the side. Only the light colors which get through the filter appear to each side of the colored part of the slit. A yellow plastic filter will usually transmit red and green while blocking the blue. A magenta filter will pass both red and blue while blocking green, and a cyan filter will pass through green and blue while blocking red.

What’s Going On?

1. When light reflects from a compact disk some is diffracted or bent to the side. Each color of light is bent through a different angle: red light is bent through the largest angle while blue light is bent through the smallest. ( see the Two-slit Interference snack for more details on how light is bent by interference.) Each position in the spectrum corresponds to a wavelength of light. So each color in the spectrum is a human perception of a single wavelength of light.

2. Transparent plastic filters absorb a wide range of frequencies of light and allow other frequencies to pass through. Yellow filters absorb blue and allow green and red to pass through. Human eyes perceive the resulting sum of green and red as the color yellow. The energy of the light absorbed by the filter, warms the filter.

3. You can also put shallow clear dishes of water, like petri dishes over part of the slit on the overhead projector and then add food coloring to the water.

Going Further

A DVD disk has a narrower track spacing than a compact disk. Try using a DVD in this exploration and compare the results.

References

This exploration was developed by Paul Doherty at project RAFT where they have an infinite supply of compact disks.