26 : 3/ Absorption Spectrophotometry 



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spectral dispersion. There is little or no inherent advantage (or dis- 

 advantage) in a grating rather than a prism for a spectrophotometer 

 used to measure the absorption of biologically interesting molecules. 



Heated 

 Filament 



Eye or Other 

 Detector 



Figure 8. Simplified diagram of a prism monochromator. 



Both can be calibrated far more precisely than has any meaning in bio- 

 logical studies, and both can be designed to pass comparable amounts 

 of light at the same wavelength bandwidth. 



A grating spectrophotometer is schematically illustrated in Figure 9 

 for a transmission type grating. Light generated by the heated filament 

 passes through slit S x located in the focal plane of lens L x . The 

 parallel light so produced falls on the grating G. Each line of the 

 grating acts as a source of light giving rise to diffracted rays which are 

 focused by the lens L 2 onto the plane of the slits S 2 . The path lengths 



Red 



Blue 

 White 



Eye or Other 

 Detector 



Heated 

 Filament 



Figure 9. Simplified diagram of a grating monochromator. 



from each successive slit to the lens differ by an amount depending only 

 on the angle 6. For most wavelengths, the light from the different lines 

 of the grating will cancel and for only one will they reinforce. In first 



