28 Light and the Eye /2 : I 



liminary part of the mating procedure in humans is based on visual 

 stimulation. Furthermore, vision acts to protect man from many 

 dangers such as those which beset him in crossing a street, driving a car, 

 or climbing the stairs. For other types of activity, vision is not necessary 

 but nonetheless plays an important role in normal human beings ; most 

 outstanding of these is the sense of balance. Finally, it should be noted 

 that human beings use visual cues more frequently than any other type 

 of sensory information. . 



Vision depends on light. During most of the evolutionary develop- 

 ment of animals, light came primarily from the sun. It is only in recent 

 times that artificial lighting has been used. Since, in their development, 

 all animals were exposed to similar physical light stimuli, it is not sur- 

 prising that all animals have similar visual ranges. This uniformity 

 contrasts sharply with the spread of the frequency ranges of hearing 

 which vary by more than an order of magnitude from one species to 

 another. 



It is necessary to understand something about the physical character 

 of visible light to have an appreciation of the phenomena of vision. 

 Light may be discussed, depending on the problem under consideration, 

 from three different avenues of approach. The first of these, and 

 historically the oldest, is called geometrical optics. It applies to many 

 problems in optics which can be solved by treating light as if it were 

 propagated as bundles of rays, each normal to the wave front. Most of 

 geometrical optics dealing with lenses can be discussed from this point 

 of view. The optical properties of the eye as a focusing lens system are 

 most simply described by geometrical optics. 



The second approach to the study of light places its emphasis on 

 wave aspects. Light waves are electromagnetic in character; the pro- 

 perties of the waves are used to describe the transmission of light through 

 a medium. In particular, the wave theories are useful in discussing such 

 phenomena as diffraction, interference, polarization, and resolving 

 power. The wave theories are also useful in discussions of visual acuity 

 and color vision. 



From the point of view of physics, the most basic approach to a study 

 of light is that of quantum mechanics. It is used in problems dealing 

 with the emission or absorption of light. In the quantum theory, light 

 is considered to be made of packets (or quanta) of energy called photons. 

 The probability of finding a photon at a given place can be described 

 by a mathematical form called a wave function. This quantum view 

 of light is necessary for studies of visual thresholds described in this 

 chapter and for the discussions in Chapter 19 of the absorption of light 

 on a molecular scale. 



The next section of this chapter presents several of the physical 



