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Diffraction Grating

A diffraction grating is an optical device consisting of many closely spaced parallel lines or grooves. In a transmission type of grating, light passes through the narrow transparent slits that lie between the dark lines on a glass or plastic plate. In a reflecting grating, light is reflected by the many parallel, narrow, smooth surfaces and absorbed or scattered by the lines cut in the reflecting surface of the grating.

During the 1870s, Henry Rowland, a physics professor at Johns Hopkins University, developed a machine that used a fine diamond point to rule glass gratings with nearly 15,000 parallel lines per inch. Today, there are carefully ruled gratings that have as many as 100,000 lines per inch. On the other hand, you can obtain inexpensive replica gratings reproduced on film with 13,400 lines per inch. To diffract very short electromagnetic waves, such as x rays, the distance between the lines in the grating must be comparable to the distance between atoms. Gratings with these small separations are obtained by using the regularly arranged rows of closely spaced ions found in the lattice structure of salt crystals.

Like a prism, a diffraction grating separates the colors in white light to produce a spectrum. The spectrum, however, arises not from refraction but from the diffraction of the light transmitted or reflected by the narrow lines in the grating. When light passes through a narrow opening, it is diffracted (spread out) like water waves passing through a narrow barrier. With a transmission type diffraction grating, light waves are diffracted as they pass through a series of equally spaced narrow openings. (A similar effect takes place if light is reflected from a reflecting grating.) The beam formed by the combination of diffracted waves from a number of openings in a transmission grating forms a wave front that travels in the same direction as the original light beam. This beam is often referred to as the central maximum.

If the light is not monochromatic, the direction of the diffracted beams will depend on the wavelength. The first order beam for light of longer wavelength, such as red light, will travel at a greater angle to the central maximum than the first order beam for light of a shorter wavelength, such as blue light. As a result, white light diffracted by the grating will form a spectrum along each ordered beam. If light from a glowing gas, such as mercury vapor, passes through a diffraction grating, the separate spectral lines characteristic of mercury will appear.

Knowing the distance between the slits in the grating and the geometry of the interference pattern produced by the diffracted light, it is possible to measure the wavelength of the light in different parts of the spectrum. For this reason, diffraction gratings are often used in spectroscopes to examine the spectral lines emitted by substances undergoing chemical analysis.

An ordinary LP record or CD, when held at a sharp angle to a light source, will produce a spectrum characteristic of a reflection grating. The narrow, closely spaced grooves in the disc diffract the reflected light and produce the interference pattern that separates light into colors. A simple transmission grating can be made by looking at the light from a showcase filament with your eyes nearly closed. Light passing through the narrow openings between your eyelashes will be diffracted and give rise to an interference pattern with its characteristic bright and dark bands.

See also Wave motion.

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