Appendix B 



Geometrical Optics 



The formulas of geometrical optics are used to describe the image- 

 forming properties of the eye. These are not different from those of 

 lenses in general. A thorough background in geometrical optics will 

 help one to understand the image-forming function of the eye. The 

 formulas describing image formation by thick lenses will be discussed in 

 this appendix. As in Chapter 2, use is made of the index of refraction 

 defined by 



»-; ■ w 



where c is the velocity of light in a vacuum and v is that in any given 

 medium. The relative index of refraction n 12 for any two media is 

 defined by 



n,o = 2S (2) 



112 " v 



i 



Before the properties of lenses are discussed, the approach of geo- 

 metrical optics is used to derive Snell's law. This proof is characteristic 

 of the general methods used. Consider a medium of index of refraction 



n bounded by a plane surface, as shown in Figure 1 . Here the line AB 

 represents the boundary between this medium and a vacuum. The 

 line aft is the normal of this surface. Now consider a plane wavefront 

 at DE at time t = 0. 



A short time later, this wavefront will have moved to D'OE'. The 

 wavefront no longer is straight since the portion of the wave in the right 



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