Lenses

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A simple lens is an optical element with perfect or approximate axial symmetry which transmits and refracts light, concentrating or diverging the beam. A compound lens is an array of simple lenses (elements) with a common axis; the use of multiple elements allows more optical aberrations to be corrected than is possible with a single element. Manufactured lenses are made of glass or transparent plastic. Elements which refract electromagnetic radiation outside the visual spectrum are also called lenses.

By analogy a catadioptric system based on a convex mirror is customarily called a lens when used in photography, but not on a telescope.