diffraction

diffraction bending of waves around the edge of an obstacle. When light strikes an opaque body, for instance, a shadow forms on the side of the body that is shielded from the light source. Ordinarily light travels in straight lines through a uniform, transparent medium, but those light waves that just pass the edges of the opaque body are bent, or deflected. This diffraction produces a fuzzy border region between the shadow area and the lighted area. Upon close examination it can be seen that this border region is actually a series of alternate dark and light lines extending both slightly into the shadow area and slightly into the lighted area. If the observer looks for these patterns, he will find that they are not always sharp. However a sharp pattern can be produced if a single, distant light source, or a point light source, is used to cast a shadow behind an opaque body. Diffraction also occurs when light waves interact with a device called a diffraction grating. A diffraction grating may be either a transmission grating (a plate pierced with small, parallel, evenly spaced slits through which light passes) or a reflection grating (a plate of metal or glass that reflects light from polished strips between parallel lines ruled on its surface). In the case of a reflection grating, the smooth surfaces between the lines act as narrow slits. The number of these slits or lines is often 12,000 or more to the centimeter (30,000 to the inch). The ruling is generally done with a fine diamond point. Since the light diffracted is also dispersed (see spectrum ), these gratings are utilized in diffraction spectroscopes for producing and analyzing spectra and for measuring directly the wavelengths of lines appearing in certain spectra. The diffraction of X rays by crystals is used to examine the atomic and molecular structure of these crystals. Beams of particles can also exhibit diffraction since, according to the quantum theory , a moving particle also has certain wavelike properties. Both electron diffraction and neutron diffraction have been important in modern physics research. Sound waves and water waves also undergo diffraction.

diffraction XVII. F. diffraction or modL. diffractio, f. diffract-, pp. stem of L. diffringere break in pieces, f. DIF- + frangere BREAK 1.

diffraction The slight bending of light around the edge of an obstacle in its path. It is a consequence of the wave nature of light. Because of diffraction, a star image consists of an Airy disk surrounded by several diffraction rings, produced by diffraction at the edge of the telescope lens or mirror. In a reflecting telescope which has a secondary mirror supported by one or more arms, diffraction around the arms causes the images of stars to have spikes. Diffraction also occurs at other wavelengths, such as radio. A point radio source occulted by the Moon, for example, disappears or reappears with brightness oscillations, rather than abruptly, as a diffraction pattern sweeps across the observatory. The size of the diffraction pattern increases with the wavelength of the radiation involved.