The process of bending of light when passing around an edge or through a slit is called diffraction. A demonstration of diffraction can be easily done by using a candle or small bright flashlight and a slit made with two pencils. The diffraction pattern (a pattern of dark and bright bands of light) which is created as the result of this experiment proves that light has wave like properties.
The aim of this science fair project is to demonstrate diffraction.
What you need?
1. Two pencils (new and of equal length)
2. A transparent tape
3. A candle
How to go about it?
1. Light the candle and make the tiny lamp to shine brightly.
2. Just below the eraser, wrap one layer of tape around the top of one of the pencils.
3. You can calculate the wavelength of light emitted by the candle or bulb, if you can measure the distances on the diffraction pattern.
4. The light should be kept at least half a meter away from you.
5. With the erasers at the top, hold the two pencils vertically. Make a thin slit between the pencils just below the tape. The skit will be formed because the pencils will be slightly apart due to the tape wrapped around one pencil.
6. Hold the pencils at a distance 1 inch away from one eye. Then you will be able to see the slit between the pencils and look at the light source through the slit. Try to make the slit smaller by bringing the pencils together. Watch the line of light perpendicular to the slit. Rotate the pencils until they become horizontal by looking through the slit. Notice that the line of light becomes vertical.
7. By looking closely to it, you can see that the light is composed of tiny bright and dark bands of light. These bands of light grow larger and spread apart when you try squeezing the slit together. You can also see it moving away from the central light source and becoming easier to see. The blobs will have blue and red edges and they will be closer to the light source.
8. Stretch a hair tight and place it about 1 inch away from you eye. Start moving the hair and hold it between your eye and the light source. Because of the hair you can see the light is spread into a line of blobs, just as it was by the slit. Now watch the line of blobs rotating by rotating the hair.
9. Watch the light through a piece of cloth, a feather, a diffraction grating, or a piece of metal screen. While looking through it, rotate each object.
The black bands between the bright bands of light show that there is a wave associated with the light. When the light passes through the slit, they spread and the light waves interfere with each other. When the crest of a wave interfere with the crest of another a bright band of light is formed whereas when a crest overlap with the trough of another wave they cancel out and as a result a dark band is observed.
The angle at which the light bends is proportional to the wavelength of the light. For instance, red light has a larger wave length than blue light and so the red light bends more than the blue light. Thus the coloured edges are produced by the different amount of bending; blue on the inside and red on outside.