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lens

lens device for forming an image of an object by the refraction of light. In its simplest form it is a disk of transparent substance, commonly glass, with its two surfaces curved or with one surface plane and the other curved. Lenses are used singly or in groups in such instruments as cameras , projectors, microscopes , telescopes , binoculars , opera glasses, and eyeglasses . The lens of the eye is known as a crystalline lens.

Classification of Lenses

All rays of light passing through a lens are refracted (bent) except those that pass directly through a point called the optical center. Lenses are classified according to the way in which they bend the rays of light entering them. Parallel rays of light passing through converging lenses are bent toward one another; these lenses are thicker at the center than at the edges. Examples are the double convex lens (both surfaces curved outward as in the simple magnifying glass), the plano-convex (one flat and one convex surface), and the concavo-convex (one surface concave, the other convex). Diverging lenses bend parallel rays away from one another; they are thicker at the edges than at the center. Examples are the double concave lens (both surfaces curved inward), the plano-concave (one surface flat, the other concave), and the convexo-concave (one surface convex, the other concave).

Design and Production of Lenses

Generally each curved surface of a lens is made as a portion of a spherical surface. The center of the sphere is called the center of curvature of the surface; every point on the surface is equidistant to it, this distance being the radius of curvature. The line joining the two centers of curvature also passes through the optical center of the lens and is called the principal axis. Any other line through the optical center at an angle to the principal axis is called a secondary axis. In converging lenses all rays entering parallel to the principal axis are bent toward a point on the principal axis called the principal focus. The distance from the principal focus to the optical center of the lens is the focal length of the lens. It varies with different lenses, according to the curvature of the surfaces and index of refraction of the lens material. Conjugate points are two points on opposite sides of a lens in such position that rays from one, after passing through the lens, will converge at the other. Light rays are not always brought to a focus at one point; this condition of inexact focus is known as aberration and may be of two types: spherical, resulting from the shape of the lens, and chromatic, resulting from the fact that different colors are refracted by different amounts (see aberration , in optics).

Lenses have long been made of glass; a piece roughly approximating the desired size and shape of the lens is cut from a glass block and then ground and polished to the correct curvature. Great skill and accuracy are required in this process and also in mounting the lenses so that the principal axes of all the lenses fall on the same line. A number of transparent plastics that permit the lenses to be cast in a mold are used as substitutes for glass.

Formation of Images

The image formed by a diverging lens is always virtual (cannot be projected on a screen as can a real image), erect (upright), and smaller than the object and is located on the same side of the lens as the object. The image formed by a converging lens depends on the position of the object relative to the focal length of the lens and the center of curvature. If the object is beyond the center of curvature, the image is real, inverted, and smaller than the object. As the object is brought toward the lens, the size of the image grows, becoming as large as the object when the object is at the center of curvature and larger than the object as the object is brought closer. When the object is one focal length away from the lens, however, no image at all is formed; and when the object moves closer than this distance, the image becomes virtual, erect, and larger than the object, as when one uses a magnifying glass.