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rods, which are kept strictly parallel to each other and at the same level. The rods carry uprights, which the optical components are positioned. A monochromatic light source such as sodium, & ur lamp illuminates a vertical slit S. Therefore, the slit § acts as a narrow linear monochromatic, o nnine The biprism is placed in such a way that its refracting edge is parallel to the length of, iheslit S. A single cylindrical wavefront impinges on both prisms. The top portion of wavefront is, fracted downward and appears to have emanated from the virtual image S,. The lower segment,, fling on the lower part of the biprism, is refracted upward and appears to have emanated from the, virtual source S,. The virtual sources S, and S, are coherent (see Fig.14.17), and hence the light, waves are in a position to interfere in the region beyond the biprism. If a screen is held ‘there,, interference fringes are seen. In order to observe fringes, a micrometer eyepiece is used., , 2p 6, , a le, , 1, 1§, 1, ", 1, , |, , , , |, , (6), , , , , , , , , , Pig. 14.17, , Theory: “_€, The theory of the interference and fringe formation in case of, Fresnel biprism is the same as described in § 14.6 for the double-slit, As, the point O is equidistant from $, and S,, the central bright fringe of, Maximum intensity occurs there. On both sides of O, alternate bright, and dark fringes, as shown in Fig.14.17(b), are produced. The width of, © dark or bright fringe is given by equ.(14.31)., , , , AD, p=, d, . Fresnel bi, Pigg Where D(= a + b) is the distance of the sources from the eyee,, lana Dark or bright fringes,, , me ye <i, , 14.9.3. INTERFERENCE FRINGES WITH WHITE LIGHT, , In the biprism experiment if the slit is illuminated by white light, the interference pattem, consists of a central white fringe flanked on its both sides by a few coloured fringes and gener, illumination beyond the fringes. The central white fringe is the zero-order fringe., , , , With monochromatic light all the bright frin:, cate the zero-order fringe. Therefore,, iluminated by white light., , u ges are of the same colour and itis not possible to, in order to locate the zero order fringe the biprism is to be, , ‘Scanned with CamScanner