Diffraction

Light can bend around edges

Material

• A minimag light (Optional Make your own.)
• two new pencils
• a hair

To Do and Notice

Look at a bright point source of light, at arms length away from you.

Look at the light in the slot between two pencils held side-by-side, close to your eye. (It helps to hold the pencils so that the eraser end of one is next to the sharpenable end of the other.
Notice that the light spreads out into a line perpendicular to the slot between the pencils, a line composed of separate blobs of light with red and blue edges.
Notice that as the pencils are squeezed together or spread apart the blobs of light spread apart and squeeze together.

Squint and look at the light through your eyelashes. Or look at the light bending around a hair stretched straight and held near your eye. Notice that a hair bends the light into a line of blobs perpendicular to the hair, just like the slit does.

What's going on

Light can bend around an edge. Red light, which has a longer wavelength, bends more than blue light. So that the blobs of light bent to the sides by the pencils have red outer edges and blue inner edges.

The diffraction pattern from a hair is identical to that of a slit of the same size. This result is called Babinet's principle.

Going Further

Look at the light through various objects such as cloth, a feather, a diffraction grating (From Edmunds), or a piece of metal mesh screen.

Etc. Isaac Newton looked at a distant candle flame while holding a pin near his eye and noted the diffraction pattern of light near the sides of the pin.

Make Your own point light source

The bulb in this flashlight is called a vacuum flashlight bulb. It is a small bulb which produces a bright point source of light and has two wire leads coming out the bottom. It can be purchased for about \$1. You can make your own point of light by connecting the flashlight leads to a radio shack double battery holder with two clip leads. Return

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 Scientific Explorations with Paul Doherty © 2002 18 September 2002