92 



MICROSCOPY 



length farther from a than from h, but again the two beams are in phase. 

 Midway between and 1, hght from a and from h are exactly out of 

 phase, with the result that they cancel each other, giving a dark spot on 

 the screen. The pattern seen on the screen, then, is a series of bright lines 

 at 0, 1, 2, etc., and also on the other side of 0, as at 2'. 



Screen 



Slits 







I 



2 



Fig. 8-7. Diffraction pattern produced by two slits. For 

 explanation see text. 



If another color of light with a longer wavelength is used, similar 

 lines appear, but the distance between and 1, and between all other 

 lines, is greater. If white light, with its mixture of all wavelengths, is 

 used, the result is a dispersed spectrum with the blue end nearer the 

 center (point 0) and the red end more remote. Each point on the screen 

 (1,2, etc.) becomes a complete spectrum. 



A diffraction grating consists of a large number of parallel lines drawn 

 on a surface. As long as the Hnes are almost perfectly parallel and the 

 spacing of the lines is uniform, the large number of lines produces a 

 diffraction pattern like that resulting from a single pair of slits. The 

 grating disperses a beam of white hght into a number of complete spectra, 

 labeled first order, second order, and so on. The preparation of diffraction 

 gratings for use in optical instruments is an elaborate process. Lines— 

 actually grooves— are engraved on a glass blank with an optically flat 

 surface by means of a very precise "ruling engine." Because the master 

 grating is extremely expensive to make, practical and useful "replicas" 



