Faced with the need to construct a large lens for a lighthouse of appropriate focal length, but unable to support the large weight of a double convex lens of that size, French physicist Augustin Fresnel (1788-1827) reasoned that it was the surface curvature which gave the focusing power. He reproduced the surface curvature of the thick lens in sections, maintaining the same focal length with a fraction of the weight. The lens strength in diopters is defined as the inverse of the focal length ...


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