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effort, by the lens-maker’s equation: (1) where n is the index of refraction of the glass of the lens, and and are the radii of curvature ofR 1 R 2 the front and back lens surfaces respectively. Often instead of specifying the focal length of a lens one quotes the power, P=1/f, of the lens. P is measured in diopters. For example a lens of 2.5 diopters has a focal length of 0.4 m. If all Refraction by a hand lens the thin lens equation pdf Greenfield A combination of two or more lenses with different indices of refraction (such as a concave lens and a convex lens) that is used to minimize a chromatic aberration. Nearsightedness A condition in which the focal length of the eye is too short to focus light on the retina.