38 



Light and the Eye \1 : 3 



cornea. Individuals lacking a lens can still see, but their vision is much 

 less sharp than that of a normal person because the image on the retina 

 is out of focus. By changing the exact shape of the lens, the eye can 

 accommodate for objects at different distances. The young person with 

 normal vision can accommodate for objects nearer than 250 mm. An 



Cornea 



Anterior 

 Focal Point 



Crystalline Lens 

 10 



17.10 



Vitreous 

 n v = 1.336 



ri a = 1.336 

 (a) 



Retina 



Principal 



Focal 



Point 



Retina 



Figure 6. Optical properties of the eye. All distances shown 

 are mm. The values are averages and will vary from indivi- 

 dual to individual. These drawings, not to scale, show Ogle's 

 modification of Gullstrand's schematic eye. Notice that 

 although the lens of the eye appears to be strong in air, it is 

 much weaker in situ since the difference in index of refraction 

 between the lens and the surrounding media is much smaller. 

 After K. N. Ogle, Optics, An Introduction for Ophthalmologists 

 (Springfield, 111.: G. C. Thomas, 1961). 



object distance of 250 mm corresponds to about 16 focal lengths. 

 Accordingly, to compensate for the change in image distance as the 

 object is moved from about 16 focal lengths to infinity, the effective 

 posterior focal length of the eye must change about 6 per cent. In 

 terms of the radius of curvature of the crystalline lens, this corresponds 

 to a change of around 20 per cent. The posterior focal length of the 



