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Optical Instruments

Lenses. - Lenses are bodies made of transparent material and bounded by faces having a cylindrical or spherical form. Although lenses differ much in form they may be divided into two classes according to the way in which they act on a parallel beam of light. Consider the lens in Figure 39a on which parallel rays are incident. Each ray is bent toward the normal to the surface on entering the lens and away from the nor­mal on emerging from the lens. In this way, the rays above the axis PO are bent downward and those below it are bent upward. After leav­ing the lens, the rays converge to a point F, called the principal focus. Such a lens is a converging lens. If the incident rays are parallel to each other, the incident wave front is a plane perpendicular to the incident rays. When this wave front emerges from the lens, it has been changed to a concave wave front that converges to the focus. When the bounding surfaces of the lens are very convex, the lens converges the rays rapidly. This gives the lens a short focal length.

a b

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