DIFFRACTION OF LIGHT, the bending of the rays of light, due to interference, as of the slits or ruled lines in a diffraction grating. The study of the diffraction of light has given us the spectrum, and the study of spectra, through the spectroscope, has added greatly to the sum of human knowledge of the constituents of heavenly bodies. A simple way of noting the diffraction of light is to darken a room and allow a ray to penetrate between the edge of a shade and the window jamb. The light falling on the opposite wall is not a well-defined streak, like the aperture through which it passes, but is divided into a series of upright streaks or bands, showing i that the rays of light bend or are dif fracted in passing through the slit. Interesting experiments with•diffraction can be made by al lowing the light to pass through a hole or holes of different forms. The phenomena of diffrac tion were first noted and commented on by Grimaldi of the Jesuits' College of Bologna in 1665. Sir Isaac Newton investigated the subject and explained it further. About 1819 Fresnel demonstrated that interference was the result of the. wave motion of light and that sound waves are similarly affected. Joseph von Fraunhofer of Munich made a most exhaustive study of diffraction and classified the lines of the spec trum, which became known as (Fraunhofer's lines' He invented the diffraction grating about 1821 for exhibiting the lines and forming the spectrum. This grating as at first made was a glass surface, smoothed as ,perfectly as possi ble and covered with a thin film of gold-leaf, lampblack, gelatine or the like. Through this
film were scratched or ruled parallel lines, con stituting minute apertures for light. In order that these lines might be made very fine, close and accurate, the dividing engine was de vised. With this were ruled a great many thou sand lines to the inch. These lines must be exactly the same distance apart to be valuable. When the light was allowed to pass through the rulings or slits of this transmission grating to the screen the light on the screen was divided or dispersed, appearing in bands and forming a spectrum. It was found that to obtain a satis factory spectrum a grating of 15,000 to 20,000 lines to the inch was desirable, and even very much finer rulings have been demanded and manufactured. In order to use gratings ruled on plane surfaces for spectroscopic purposes it is necessary to employ a telescope and colli mator. After a time the reflecting grating was invented. In this a surface of spectrum metal was ruled, and this had some advantages over glass. Prof. Henry A. Rowland of Baltimore achieved distinction in the production of im proved diffraction gratings, making a reflecting grating with a concave surface that can be used without lenses. The diffraction grating has been much used in studying the solar spectrum. See LIGHT ; SPECTRUM ; SPECTROSCOPE.
For further' information as to diffraction gratings conslt 'Gratings in Theory and Prac tice (in and Astcs,) Vol. XII, 1893). For information as to diffraction consult Preston, 'Theory of Light' (London 1901).