GENERATION, CONTROL, AND MEASUREMENT 



203 





MIXTURE OF 

 400-mjU AND 



800-m^ RAYS 

 Sin / =0 



I 



(b) TRANSMISSION GRATING 





NORMAL TO 



GRATING -jy 



(c) ECHELETTE REFLECTION GRATING 



Fig. 3-22. (a) The 60° prism, (6) the transmission grating, and (c) the echelette 

 reflection grating as dispersive elements. The angles i and d refer to angles of inci- 

 dence and refraction (a) or diffraction (c), respectively. Diagram a shows how blue 

 and red rays are refracted and dispersed in passing through a prism. Diagram b, for 

 the diffraction grating, shows how the various orders of blue and red flux are diff'racted 

 on both sides of the zero-order or undiffracted image. In the echelette grating, shown 

 in c, the most intense portion of the spectrum (the blaze) is obtained when the bottom 

 of the groove is tilted at the groove angle a so that the angle of groove incidence 

 a + a) of the rays to the bottom of the groove is equal to the angle of reflection 

 (e — a). In the region of the blaze the rays are reflected from the bottom of the 

 groove as in a simple mirror. The grating spacing is d. 



where i is the angle of incidence at minimum deviation and n is the 

 refractive index. The quantity dn/d\ is a complex function of wave 

 length, the form of which varies greatly with different materials. 



The most common prism materials for the visible and ultraviolet are 

 glass and quartz, and for the infrared, the halide crystals, such as sodium 

 chloride and potassium fluoride. 



