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Piece of glass or other transparent substance that is used to form an image of an object by converging or diverging rays of light from the object. Because of the curvature of its surface, different rays of light are refracted (seerefraction) through different angles. A convex lens causes rays to converge on a single point, the focal point. A concave lens causes rays to diverge as though they are coming from a focal point. Both types cause the rays to form a visual image of the object. The image may be realinverted and photographable or visible on a screenor it may be virtualerect and visible only by looking through the lens.
This entry comes from Encyclopædia Britannica Concise. For the full entry on lens, visit Britannica.com.