2 : 3/ Light and the Eye 



37 



techniques indicate that the vitreous humor does contain some sort of 

 structure. Optically, the vitreous humor is indistinguishable from the 



Wide 



Aperture 



Light rays 



proceeding so as 



to focus to a 



point at q 



iris 

 Diaphragm 



(a) 



Retina 



Light rays 

 as above 



Narrow 

 Aperture 



(b) 



Iris 

 Diaphragm 



Retina 



Figure 5. Effect of aperture on depth of focus. A point 

 focused at q will appear as a circle of diameter 8 on the retina, 

 As shown in (a), if the aperture of the iris diaphragm is wide, 

 the diameter of 8 will be large; hence, one image will blur into 

 the next unless q is very close to the retina. Thus, increasing 

 the aperture decreases the depth of focus. As shown in (b), a 

 narrower aperture increases the depth of focus but decreases 

 the luminous energy reaching the retina. 



aqueous humor which fills the space in the eyeball between the cornea 

 and the crystalline lens. The aqueous humor, as its name implies, is a 

 water-like solution containing the normal solutes of a body fluid. 



B. Geometrical Optics of the Eye 



Light enters the eye through the transparent cornea. It then passes 

 through the aqueous humor, through the crystalline lens, and into the 

 vitreous humor. It is received on the photosensitive retina, where there 

 must be an image in focus if the object is to be seen clearly. The 

 dimensions, radii of curvatures, distances apart, and positions of the 

 six cardinal points are shown in Figure 6 for a schematic eye. 



The greatest part of the refractive power of the eye occurs at the 



