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# Diffraction of Light

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Source:
Author: Janice VanCleave

So You Want to Do a Project about Diffraction!

### Purpose

To observe diffraction of light.

### Materials

• Straight pin
• Index card
• Pen
• Yardstick (meterstick)
• Desk lamp

### Procedure

1. Use the pin to make a very small pinhole in the center and near the edge of one of the short ends of the index card.
2. Use the pen to draw a circle around the hole so that the hole is easy to find.
3. Fold the index card in half with the short ends together.
4. Partially unfold the card so that the two sides are perpendicular to each other.
5. Hold the card so that the end with the hole is in front of your face and the other end touches the tip of your nose.
6. Stand about 6 feet (1.8 m) or more from the desk lamp.
7. Close one eye and with the open eye look through the pinhole at the bulb of the desk lamp. What do you see inside the hole?

### Results

You should see dark bands across the light in the hole.

### Why?

Light is a form of energy that travels in transverse waves similar to water waves in shape. Light waves are disturbances that can travel through space in a regular pattern. Transverse waves have a crest (the highest point of a wave) and a trough (the lowest point of a wave). The distance from any point of one wave to the same point of the next wave is called wavelength.

When you look at the light through the hole in the index card, some of the light passing through the tiny hole spreads out instead of traveling straight ahead. This behavior of light is called diffraction (the change of direction of a ray of light around the edge of an object or through a small hole). The light that passes through the center of the hole goes straight, but light hitting the edge of the hole is reflected. Each point on the opening that reflects light acts as a source of light. When rays of light from different sources meet each other, they create bands of dark and light. This behavior of light is caused by the rays' wave shapes.

When two overlapping light waves meet and their crests and troughs match, the light energy adds together and the light is brighter. This is called constructive interference. But when the crests and troughs of the overlapping waves are opposite, the light energy is canceled, causing a dark area. This is called destructive interference. Despite the name, interference has no effect on the waves themselves, just on the way we see them. After meeting, the waves continue to move as they did before they met.

### For Further Investigation

When light from a distant star passes through the aperture of a telescope, the light is diffracted. Does the size of a telescope affect how it diffracts light? A project question might be, How does the aperture of a telescope affect light diffraction?

1. Take the card from the original investigation and make several additional holes in it, each one a bit larger than the last, with the largest hole being about the size of a pencil point. Repeat the experiment, comparing the amount of interference in each hole. The clearer the light that passes through the hole, the less diffraction and hence the less interference.
2. From your results, determine which would produce more resolved (less blurred) images-a telescope with a large aperture or one with a small aperture.

### References and Project Books

Burnie, David. Eyewitness Books: Light. New York: Dorling Kindersley, 1999.

Gardner, Robert. Science Projects about Light. Springfield, N.J.: Enslow, 1994.

Glover, David. Sound and Light. New York: Kingfisher Books, 1993.

VanCleave, Janice.]anice VanCleave's Physics/or Every Kid. New York: Wiley, 1991.

Wiese, Jim. Roller Coaster Science. New York: Wiley, 1994.

Williams, Brian. Science and Technology. New York: Kingfisher Books, 1993.

Wood, Robert W. Light Fundamentals: Funtastic Science Activities for Kids. New York: Learning Triangle Press 1997.