DIFFRACTION AND DIFFRACTION GRATINGS (front Lat. diffringure, to break in pieces, from dis-, apart frangcre, to break). When shadows ale cast on a screen by allowing light from a small opening or source to pass an opaque body. it is noted that the shadows are not sharp. as would be expected if 'light traveled in straight lines.' Thus. if light from the sun or from any source of light falls upon a large opaque screen in which there is a narrow slit; and if there is placed some distance from this another opaque body with a sharp edge ie. g. a knife-blade) parallel to the slit, the shadow of the latter cast by the light from the slit will have several peculiarities. Calling the geometri cal shadow the region where there would he darkness if light traveled in straight lines, it will be observed that the actual shallow begins a slight distance within the geometrical one—in other words. the light is bent slightly into the latter; further, on the edge of the geometrical shadow, where one might expect uniform illumi nation. there are series of narrow colored hands. parallel to the edge. that is, there is first a strip brightly illuminated with one color, then an other hand with a different color, etc.. the colors merging into each other: at a distance of a few millimeters from the edge of the shadow, how ever, the field is uniformly illuminated. If in stead of having white light as the source, a col ored flame is used. e. g. light from a flame con taining sodium. the bands mtside the geometri cal shadow become simply alternations of light of the colors of the flame, and darkness. This phenomenon is called 'iliffraetion.' and was first observed by Grimaldi. professor in the Jesuits' College, Bologna. in 11415. It was studied later by Newton, but twos not explained until Fri made his classical research ice 1819. Similarly. if light from a pin-hole open ing falls upon any opaque object. casting a shadow of it on sane suitable screen, diffraction hands may he observed outside the geometrical shadow, following the general contour of the latter; and the light enters slightly into the shadow-. For other cases of diffraction. reference
should be made to sonic treatise on light. flue of the best is Preston's Theory of Light Ilmmion. MI5).
It was shown by Fresnel that diffraction phe nomena a re explained by the fact that light is doe to a wave-motion (see Licicr) : consequently, diffraction is possible with other waves. e, g. !hose in air which prnance the sensation of sou nd.
The colors of the feathers of many birds and of 'mother-of-pearl' are due to diffraction; so are the colors seen when one looks at a bright light through a piece of thin cloth. If a great number of tine scratches. evenly spaced. are made on a piece of glass. or on a polished mirror, there is formed a 'diffracting grating.' The former is called a 'transmission' grating; the latter. a re flecting one. lf, as in a previous experiment, 1,vhite light. from a slit in an opaque screen is allowed to fall perpendicularly upon a trans mission grating. the slit being parallel to the scratches. the transmitted light will be broken up into spectra. regularly spaced on both sides of the line drawn from the slit to the grating, ex cept in the direction of this line. Thus, if the transmitted light falls upon a white screen paral lel to the grating, there will lie a central white spot, and On each side of this. along a line at right angles to the lines on the grating, a suc cession of colored spectral bands. This spreading sidewise of the light is due to its diffraction through the slits between the scratches: and the dispersion of the while light is occasioned by the fact that the amount of diffraction varies with waves of different wave-number. i. e. with different colors (see LIGHT). The same phenomena are observed with reflection gratings if the screen is placed on the same ride as the slit of light and provided with an opening to allow the light to fall upon the grating.