lens

The passage of light through lenses. The concave lenses diverges a beam of light from a distant source. The convex and compound lenses focus light from a distant source to a point. The distance between the focus and the lens is called the focal length. The shorter the focus, the more powerful the lens.

A convex (or converging) lens causes light rays to refract inwards. A concave (or diverging lens) causes light rays to refract outwards. Convex lenses are used to correct long-sightedness and concave lenses to correct short-sightedness.

In optics, a piece of a transparent material, such as glass, with two polished surfaces - one concave or convex, and the other plane, concave, or convex - that modifies rays of light. A convex lens brings rays of light together; a concave lens makes the rays diverge. Lenses are essential to spectacles, microscopes, telescopes, cameras, and almost all optical instruments.

The image formed by a single lens suffers from several defects or aberrations, notably spherical aberration in which an image becomes blurred, and chromatic aberration in which an image in white light tends to have coloured edges. Aberrations are corrected by the use of compound lenses, which are built up from two or more lenses of different refractive index.

Lens

Former coal-mining town in Pas-de-Calais département, northern France; population (1990) 35,300, conurbation 327,000. Present-day industries include chemicals, metallurgy, and textiles. The area around Lens was a important theatre of operations in World War I, particularly during the Battle of Arras. By 1918 the town lay in ruins, but was rebuilt. Lens was again damaged in World War II.

At nearby Vimy is a memorial to Canadian troops who fought in April 1917 at the Battle of Vimy Ridge in World War I.

History Lens was the scene of a victory by the Prince of Condé (the Great Condé) in 1648 during the Thirty Years' War. During World War I Lens was occupied by the German army and lay close to the front line from October 1914 to October 1918. The town and its mines were so severely damaged that mining operations could not restart until 1921. In World War II it was occupied by Germany from May 1940 to September 1944, but suffered less physical damage.