Fig. 85. In the case of a convex lens, fig. 86, the parallel rays aremade to converge by the lens. The point F is on the opposite … Fig. 86. side of the lens to the distant source. The principal focus isreal and the focal length AF is negative. Hence The focal length of a concave lens is positive, that of aconvex lens is negative. The relation between the focal length of a lens, the form of itssurfaces and its refractive index…
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