Diffraction

Diffraction of light differs from diffraction of sound because diffraction is most evident when the obstacle is about the same size as the wavelength diffracted. The sound waves we hear have wavelengths of about a yard and are diffracted by ordinary objects. But visible light waves have wavelengths of less than 1/35,000 of an inch (0.00007 centimeter). Thus, light waves can be diffracted noticeably only by extremely small objects.

How diffraction occurs. Diffraction takes place among all waves at all times. To understand why it becomes noticeable only when the obstacle is about the size of the diffracted wavelength, one must understand both diffraction and interference.

Christiaan Huygens, a Dutch scientist, developed the principle that explains why diffraction occurs. This principle states that each point on the surface of a wave is the source of small waves. These wavelets move outward in all directions. To find the total wave reaching an area, all the wavelets that strike the area must be considered. If the crests of two wavelets reach a point at the same time, they reinforce each other. This condition is called constructive interference, and the resulting wave is large. If the crest of one wave reaches a point at the same time as the low point of another, the two waves cancel each other. This condition is called destructive interference, and the resulting wave is small or nonexistent. See Interference.

A beam of light moves in a straight line because effects of diffraction outside the beam are canceled by destructive interference. The wavelets at the edge of the beam spread, but most of the light travels in a straight line with the beam. When light travels through a tiny opening, interference occurs only among the wavelets coming from the opening. These wavelets produce a diffraction pattern because most of the destructive interference has been eliminated.

Diffraction of light from a tiny source can likewise be observed if some of the light--and thus its interference--is removed. A disk placed in the path of such a source blocks out the wavelets that originate behind the disk. At points beyond the disk, these eliminated wavelets are missing not only in the shadow of the disk but also outside of the shadow, where they would have interfered constructively. The shadow pattern on a screen beyond the disk consists of a series of rings, alternately light and dark, in and surrounding the shadow area. A bright spot occurs at the center of the shadow because at that point all waves interfere constructively. They do so because they have all traveled the same distance from the edge of the disk.

Uses of diffraction. The occurrence of diffraction has been used as a test of whether various things are waves. For example, diffraction of X rays by crystals convinced scientists that X rays are waves.

The pattern of X-ray diffraction depends on the type and distribution of atoms in the diffracting substance. This fact has been used to study the structure of crystals by X-ray diffraction and to discover the structure of proteins and nucleic acids.

A diffraction grating is a glass plate with lines ruled on it at small, equal intervals. Light can pass only between the lines, and the slits are about as far apart as a wavelength of light. If a parallel beam of white light strikes the grating, a pattern of light of various colors appears on a screen beyond the grating. The colors appear because white light consists of different colors. These colors have different wavelengths, and the longer wavelengths are diffracted at greater angles. Scientists can identify a substance by the pattern of colors it produces through a diffraction grating.

Contributor: Gerald Feinberg, Ph.D., Former Prof. of Physics, Columbia Univ.

See also Light; Molecule; Sound; Spectrometer; Waves.

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