Farmasevtika instituti farmakologiya va klinik farmatsiya kafedrasi

Focal Length of a Lens 
The distance beyond a convex lens at which 
parallel 
rays converge to a common focal point is 
called the 
focal length 
of the lens. The diagram at the top of Figure 49-6 demonstrates this focusing of 
parallel light rays. 
 
Figure 49-5 
A, 
Focusing of light from a point source to a line focus by a cylindrical lens. 
B, 
Two cylindrical convex lenses at rightangles to each other, demonstrating that one lens 
converges lightrays in one plane and the other lens converges light rays in theplane at a right 
angle. The two lenses combined give the samepoint focus as that obtained with a single 
spherical convex lens. 
Focal pointsLight from distant source Point source
Figure 49-6 
The 
two upper lenses of this figure have the samefocal length, but the light rays entering the top 
lens are parallel, rays on the focal distance is shown. 
Measurement of the Refractive Power of a Lens
—―Diopter‖ 
The more a lens bends light 
rays, the greater is its ―refractive power.‖ This refractive power is measured in terms of 
diopters. 
The 
refractive power in diopters of a convex lens is equal to 1 meter divided by its focal length. Thus, a 
spherical lens that converges parallel light rays to a focal point 1 meter beyond thelens has a refractive 
power of +1 diopter, as shown in Figure 49-8. If the lens is capable of bending parallel light rays twice 
as much as a lens with a power of +1 diopter, it is said to have a strength of +2 diopters, and the light 
rays come to a focal point 0.5 meter beyond the lens.
Optics of the Eye The Eye as a Camera 
The eye, shown in Figure 49-9, is optically 
equivalent to the usual photographic camera. It has a lens system, a variable aperture system (the 
pupil), and a retina that corresponds to the film. The lens system of the eye is composed of four 

---

refractive interfaces: (1) the interface between air and the anterior surface of the cornea, (2) the 
interface between the posterior surface of the cornea and the aqueous humor, (3) the interface between 
the aqueous humor and the anterior surface of the lens of the eye, and (4) the interface between the 
posterior surface of the lens and the vitreous humor