5 



CHAPTER L-v Light Absorption Effects 



INTRODUCTION 



Quantum physics teaches us that we may talk about radiant energy as 

 traveling either as waves in space or as discrete packages called either 

 photons or quanta. When radiation impinges on matter, it has a proba- 

 bility of being absorbed which depends on (1) the energy in the photon 

 (or its equivalents in wave terms: the wavelength or the frequency of the 

 light), and (2) the nature of the matter. From the photon view, the 

 photon may be absorbed if and only if there are two energy states 

 (roughly two electron orbits) whose energy difference corresponds exactly 

 to the photon energy, as sketched in Fig. 22. And, of course, there must 

 be an electron in the lower of the two states, for there must be some 

 particle to absorb the energy. If the structure of the matter is such that 

 there is no energy difference equal to that of the impinging photon, the 

 photon may impart its energy to the electron, but the electron will 

 rapidly return to its original state and reradiate the energy. Thus, in 

 effect, this kind of matter doesn't absorb the energy at all; because of the 

 time taken for the absorption and re-emission, however, the reradiated 

 photon is delayed relative to its initial path, and we say that its phase 

 has been shifted. 



Substances (such as glass) in which the energy gap between the elec- 

 trons and the next higher energy state is so great that virtually none of 

 the incident energy of visible light is absorbed, make suitable insulators. 

 Electrical conductors, on the other hand, are composed of substances in 

 which, by definition, the electrons are free to move, which means that 

 there are so many permissible energy states that almost every incident 

 photon would be absorbed. In advanced physics texts it is shown why 

 the photons re-emitted by electrons in metals are virtually all in the 

 backward direction. The result is that even a thin sheet of metal (e.g., 

 household aluminum foil) is quite opaque, because the incident light is 

 reflected backward. 



When light is absorbed, the energy does not remain very long in the 

 absorbing substances. The fact that the colors of substances do not 

 change during illumination may be taken to mean that the absorbing 

 substances quickly revert to their original state, so that they arc 1 once 

 again ready to absorb the same wavelengths. There 1 are five main fates 



52 



