Earth science

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Video

Materials

• 1 strong directional lamp (the stronger and whiter the better)
• 1 CD
• 1 dark bed sheet

Short explanation

White light from the Sun or a lamp actually consists of many colors. When white light hits a CD, it separates and the individual colors become visible.

Long explanation

On a CD, the information is stored in the form of rows of elevations on an otherwise flat "landscape". The distance between these elevations is very small, so small that it's in the same order of magnitude as the wavelength of light. This causes something strange to happen when light hits the surface of a CD. It's divided into the different colors that the light consists of. Pure white light consists, for example, of all possible colors and these are reflected in different directions on the CD's bumpy surface, at the same time as they are amplified, and become clearly visible.

In this demonstration, the CD acts as a diffraction grating, which is a technical device used to separate different colors.

You can see this rainbow of colors just by looking at the CD. Your eyes are hit by the reflected light. But you can also let this light travel further to a white roof, and get an even more beautiful view.

To make the rainbow clearer, it's good idea to cover all surfaces that only reflect the white light without dividing it into colors.

A rainbow in the atmosphere is also created by the separation of colors at the same time as they are reflected. But in the sky it happens a little differently - not by the light being reflected on a landscape of tiny irregularities, but by the light moving from one medium to another. Namely, a rainbow arises with the help of small water droples in the air. When a white light ray enters the "front" of a droplet of water, and passes the transition between air and water, the different colors refract in different directions. The light ray changes from being narrow and white to becoming a little wider and consisting of all the colors in a rainbow. The light ray is then reflected on the inside of the "back" of the water droplet (some light also passes out here, but doesn't give rise to a rainbow) and turns back approximately towards the direction of the Sun. Then the light ray again crosses the boundary between water and air on the front of the water droplet, though this time on its way out of it, and the colors are separated even more. The light ray is spread to some extent in all directions from the front of the water drop, but most of its colors are concentrated 40-42° from the direction of the Sun. If you want to see a rainbow, you should stand with the Sun in your back and look at water droplets in the sky.

CDs are a bit different in how the are made and therefore you should try with several different ones. Some give clear rainbows, while others more diffuse.

Experiment

You can turn this demonstration into an experiment. This will make it a better science project. To do that, try answering one of the following questions. The answer to the question will be your hypothesis. Then test the hypothesis by doing the experiment.

• What happens if you change the distance between the CD and the lamp?
• What happens if you change the distance between the lamp/CD and the ceiling?
• What happens if you change the angle of the lamp?
• What happens if you cover half the CD with something dark?
• What happens if you bend the CD?
• What happens if you change the type of lamp?
• What happens if you use multiple lamps?

Variation

Try to place several CDs of the same type at the light source, so that they reflect their rainbows in the same place. These rainbows then reinforce each other into a stronger rainbow. This requires a strong and wide light source. Here, a ray of sunlight that is let into an otherwise darkened room may be best.

Gilla:		Dela: