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Absorption Spectrophotometry /26 : 3 



the light so that the beams of any given angle reach a spot in the plane 

 of slits S 2 . The slits permit only one narrow wavelength band to pass, 

 thus giving rise to monochromatic light. By rotating the prism about 

 an axis perpendicular to the plane of Figure 8, light of different wave- 

 lengths can be brought to S 2 . The monochromator can thus be adjusted 

 to any desired wavelength. 



Figure 6. Minimum angle of 



deviation for light refracted by a 



prism. Most elementary physics 



texts show that if A is small 



O = (n - I) A 



A 

 H = Yi = 2 



(nA) 



h = y 2 = — 



where n is the index of refraction. 



Figure 7. Refraction by the prism 

 illustrated in Figure 6. A differ- 

 ent wavelength gives a greater 

 deviation. 



In general, there is not a simple relationship between the angle of 

 deviation and the wavelength. The dial adjusting the prism (and 

 hence, the wavelength selected) must be calibrated for the substance 

 used to form the prism. The purity of the light is controlled by width of 

 the slits S x and S 2 . For most prisms the slit width W is related to the 

 band width passed by a relationship approximately given by 



AA/A = kW 



where k is a constant independent of wavelength or slit width. In other 

 words, for fixed slit width, the fractional or logarithmic bandwidth is 

 approximately constant. 



A monochromator for which the wavelength calibration depends on 

 geometry only, and for which there is a constant bandwidth, that is 



A = k'S 



can be constructed by using a grating in place of the prism to provide 





