2 : 2/ Light and the Eye 



31 



the focal length and decrease the lens strength. In a system of a series 

 of spherical surfaces, such as is found in the eye, the forward and back- 

 ward focal lengths will be different. 



B. Light as an Electromagnetic Wave 



Although many actions of lens systems may be adequately described by 

 geometrical optics, others cannot be. In the last chapter, reference was 

 made to the phenomena of diffraction and interference. Diffraction 

 refers to the fact that a wave will not behave as a bundle of rays, especially 



Resolvable as 

 Two Images 



"Limit of 

 Resolution ' 



Not Resolvable as Two Images 

 According to Rayleigh Criterion 



Figure 2. Dual Diffraction Patterns. 



in the neighborhood of objects comparable in linear dimensions to the 

 wavelengths of the light. (See Chapter 1 for a definition of wavelength.) 

 In discussing sound, it was noted that the wavelengths of many audible 

 sounds were comparable to the sizes of rooms and buildings. Thus, 

 speech sound waves are diffracted by (or bent around) the furniture 

 and other objects. The wavelength of visible light is much smaller than 

 most common objects; hence, diffraction effects are not a usual part of 

 everyday experience. However, experiments with slits, fine wires, 

 small spheres, and so forth show that diffraction effects do occur. For 

 similar reasons, interference effects in the form of standing waves are 

 familiar in sound experiments but demand special equipment in order 

 to be demonstrated for light. These and many other experiments make 



