Diffraction is the bending of waves (such as light waves or sound waves) as they pass around an obstacle or through an opening. Anyone who has watched ocean waves entering a bay or harbor has probably witnessed diffraction. As the waves strike the first point of land, they change direction. Instead of moving into the bay or harbor parallel to (in the same direction as) land, they travel at an angle to it. The narrower the opening, the more dramatic the effect. As waves enter a narrow harbor opening, such as San Francisco's Golden Gate, they change from a parallel set of wave fronts to a fan-shaped pattern.
The diffraction of light has many important applications. For example, a device known as the diffraction grating is used to break white light apart into its colored components. Patterns produced by diffraction gratings provide information about the kind of light that falls on them.
All waves are subject to diffraction when they encounter an obstacle in their path. Consider the shadow of a flagpole cast by the Sun on the ground. From a distance the darkened zone of the shadow gives the impression that light traveling in a straight line from the Sun was blocked by the pole. But careful observation of the shadow's edge will reveal that the change from dark to light is not abrupt. Instead, there is a gray area along the edge that was created by light that was bent—or diffracted—at the side of the pole.
When a source of waves, such as a lightbulb, sends a beam through an opening, or aperture, a diffraction pattern will appear on a screen placed behind the aperture. The diffraction pattern will look something like the aperture (perhaps a slit, a circle, or a square) but it will be surrounded by some diffracted waves that give it a fuzzy appearance.
The diffraction that occurs depends primarily on two variables: the wavelength of the wave and the size of the opening or aperture through which the waves pass. (Wavelength is defined as the distance between two identical parts of a wave, such as two consecutive crests of a wave. The only difference between waves of light, waves of radar, waves of X rays, and of many other kinds of waves is their wavelength—and their frequency, which depends on their wavelength.) The wavelength of light, for example, is in the range of 400 to 700 nanometers (billionths of a meter). In comparison, the wavelength of radar waves ranges from about 0.1 to 1 meter.
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