Light, Diffraction Of
It is commonly thought that light travels in a straight line. In fact, light is a wave, and, like all waves, it can be made to bend around corners and obstacles. This bending of light is called diffraction. Diffraction is a phenomenon found in all waveforms. It is what causes sound to be heard around corners and waves to fill an enclosed harbor. As a wave encounters an obstacle, a corner, a sharp edge, or a narrow aperture, a new wave begins from that point and radiates out in a circular pattern.
The first person to recognize diffraction was the seventeenth century Italian scientist Francesco Grimaldi. He noticed that shadows do not have sharp edges; rather, they possess bright lines just within the shadow's edges. As he looked more closely he found a series of fainter lines that became dimmer as their distance from the edge increased. He correctly presumed that light had bent around the object casting the shadow.
Scientists passed light through a series of narrow slits in order to better understand diffraction. When the light exited the slit it emerged as a new waveform; if two slits were placed close together, thewaveforms would interact. At certain points the waves amplified each other, producing a bright line; at other points the waves cancelled each other out, creating a dark spot. The resulting pattern of alternating bright and dark lines is called an interference pattern. For most purposes, interference and diffraction can be thought of as essentially the same thing.
There are generally two types of diffraction. If the light source is very distant (such as starlight), the individual light rays will be parallel to each other; this effect can also be achieved by focusing the light through a lens. The diffraction of parallel light rays is called Fraunhofer diffraction, after the German scientist Joseph von Fraunhofer (1787-1826). If the light source is relatively close and unfocused, the light rays will not be parallel. This kind of diffraction is known as Fresnel diffraction, after the French scientist Augustin-Jean Fresnel.
As they continued to experiment, scientists soon noticed that the diffraction patterns they obtained were streaked with color--in fact, they contained the complete color spectrum. It was determined that longer wavelengths (such as red light) are diffracted more sharply than shorter wavelengths (such as blue light), thus the light can be separated into its composite colors, a process called dispersion. The amount that light is dispersed is determined by the width of the slit the light passes through; the narrower the slit, the greater the dispersion.
Physicists realized that the best method for observing the behavior of light was to design an instrument that would produce a very wide, very bright spectrum. It should combine a sharp interference pattern with broad dispersion. What they came up with was the diffraction grating.
A diffraction grating is a piece of optically flat glass that is scored with many thin slits. The spacing of the slits depends upon the kind of light that is to be dispersed, and there can be as many as 30,000 slits per 1 inch (2.5 cm). The light is either allowed to pass through the grating or, if an aluminum or silver coating is applied, to reflect from it. A quality grating will produce a very precise and easily observed spectrum.
Because diffraction affects all waveforms, many types of diffraction gratings have been produced; the longer the wavelength, the farther apart the slits must be spaced. A grating designed to diffract low-energy microwaves would have slits that are several centimeters apart. X-rays, on the other hand, are much higher in energy and possessing very short wavelengths; no machinery yet invented could produce a grating whose slits were spaced close enough to diffract them. However, Max von Laue hypothesized that the regular molecular structure within crystals would be the perfect natural grating for X-rays. The diffraction of X-rays by crystals has evolved into a new science, X-ray crystallography.
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