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Light passing through a lens is actually bent at each surface. However, in simple ray diagrams it is possible to disregard this and draw all the bending at a single line (the principal plane) running down the center of the lens.
A lens is a piece of transparent material having at least one curved surface and which is used to produce an image or otherwise focus light by refraction. Lenses are found in cameras, glasses, microscopes, telescopes, and other optical instruments. The typical thin lens is formed from a glass disk, though crystalline minerals and molded plastics are also used and, as with spectacle lenses, shapes other than circular are quite common.

The principal axis of a lens is the imaginary line perpendicular to its surface at its center. Lenses which are thicker in the middle than at the edges focus a parallel beam of light traveling along the principal axis at the principal focus, a point on the axis on the far side of the lens from the light source. Such lenses are converging lenses. The distance between the principal focus and the center of the lens is known as the focal length of the lens; its focal power is the reciprocal of the focal length and is expressed in dioptres (m-1).

A lens thicker at its edges than in the middle spreads out a parallel beam of light passing through along its principal axis as if it were radiating from a virtual focus one focal length from the lens center on the same side as the source. Such a lens is a diverging lens.

Lens surfaces may be either inward curving (concave), outward bulging (convex), or flat (plane) and it is the combination of the properties of the two surfaces which determines the focal power of the lens.

A convex lens (1) focuses parallel rays of light on a single point (2), whereas a concave lens causes the rays to diverge. A magnifying glass (4) is a convex lens that alters the path of light rays so that they appear to come from a larger image (5) than is the actual case. Cameras use a combination of convex and concave lenses (6) to focus light without separation of the colors of the spectrum.

In general, images of objects produced using single thin lenses suffer from various defects including spherical and chromatic aberration, coma (in which peripheral images of points are disturbed into pear-shaped spots), and astigmatism. The effects of these are minimized by designing compound lenses in which simple lenses of different shapes and refractive indexes are combined. Achromatic lenses reduce chromatic aberration; aplanatic lenses reduce this and coma, and astigmatic lenses combat astigmatism.

A lensmeter is an instrument used to measure the refractive properties of lenses.

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