Magnifying Glass from a Hand-Ground Lens

A magnifying glass is a convex lens in a frame with a handle. A typical one has a focal length of 25 cm, an optical power of 4 dioptres, and is sold as "2×": its magnifying power is 0.25 m × 4 + 1 = 2 for an eye whose near point is 25 cm. Here the lens is ground plano-convex from crown glass (refractive index about 1.52). For a plano-convex lens the focal length is R / (n − 1), so a 25 cm focal length needs a curve of radius about 13 cm. The lens is then set in a turned wooden ring with a handle; those sizes are ours.