Focal Length If parallel rays of light in a fast medium encounter a hemispherical interface to a slower medium, they will be redirected to converge more or less on a single focal point . While the rays do not all perfectly converge on the same point—because a sphere is not the ideal curve for this purpose, and in reality there is no ideal shape for focusing rays under all conditions—spherical surfaces are easy to model and physically manufacture, and work well enough for most applications. The curved shape just described qualifies as a lens . It naturally has a radius of curvature since it is part of a sphere, and it also has a focal length defining how light rays interact with it. In fact, we find that the focal length f also depends on both refractive indices involved and on whether the center of curvature is to the right or the left; parallel rays diverge at a reversed interface as if they had come from an imaginary focal point on the left side. Most often, the lens is more than a single interface; it is a pair of curves enclosing a
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This note was uploaded on 04/05/2012 for the course PHYS 131 taught by Professor Tibbets during the Spring '11 term at Cuyamaca College.