ACTION SPECTRA 65 



Fig. 29. Light passing through a prism is broken up into the spectrum as 

 shown. A container C of finite size will necessarily intercept a whole range of 

 wavelengths. 



For each of these functions an action spectrum can be obtained, and 

 there is no reason to expect the same spectrum for all effects; in fact, 

 different spectra are found. Therefore it is possible to learn some struc- 

 tural and functional lessons from action spectra of this kind, too. 



The obtaining of an action spectrum is not as simple as it may seem, 

 due entirely to practical considerations. We have spoken as though it is 

 possible to irradiate with monochromatic light, i.e., light of a single 

 frequency. In practice, the source of light is some high-intensity lamp 

 whose light is used in one of two ways. As indicated in Fig. 29, the light 

 may be passed through a diffraction grating or through a prism to spread 

 the light out into a spectrum. If the container C being irradiated is 

 relatively large, or is placed relatively close to the spectrum-producing 

 device, it will intercept light of many wavelengths, thereby producing an 

 effect composed of all the wavelengths absorbed. If the container is 

 placed so far away from the source that it intercepts only a narrow 

 range of wavelengths, the total energy received becomes very low, be- 

 cause the intensity falls off inversely with the distance. Thus there is a 

 practical limit to the minimum range of wavelengths being absorbed. 



A second device is a so-called interference filter which, as its name 

 suggests, uses an interference method to let through only a very small 

 range of wavelengths. This filter may be placed very close to an intense 

 light source to provide a high intensity. However, the band of wave- 

 lengths transmitted by such a filter is great enough to give trouble with 

 interpretation in terms of a single wavelength. Thus, in practice, action 

 spectra taken with a finite number of wavelengths are only roughly 

 accurate, so that only broad classes of compounds can be designated as 

 being involved in the action being studied. Happily, this is frequently 

 all the information that is desired. 



