I40 PHYSICAL SCIENCE 



The usual form of diffraction grating consists 

 of a transparent or reflecting surface, on which 

 a large number of parallel scratches are ruled 

 very accurately, so near together that the distance 

 between them is comparable with the wave-length 

 of light. By allowing light to fall on such a 

 surface, a spectrum is formed, like that given by 

 a prism or a rainbow. A similar effect could be 

 obtained by a number of very thin glass plates, 

 P P P P ^^ F ^S- 29, piled closely one upon another. 

 Let A A' A'' N'^ denote a wave-front of homo- 



FiG. 29. 



geneous light, such as the yellow rays from a 

 colourless gas flame in which a sodium salt is 

 placed. This light is reflected at B B' B^' B^^ 

 and, in one particular direction BC, all these 

 reflected rays coalesce. If BC is in such a 

 direction that the difference in path between 

 ABC and A' B' C is just one wave-length, the 

 crest of one wave will coincide with the crest of 

 the next. All the little waves, therefore, produce 

 similar effects, and the resultant effect is large — 

 a bright yellow line appears along BC. Else- 

 where there will be no such coincidence. Crests 

 and troughs of the wavelets mix together, inter- 



