lens

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A ground or molded piece of glass, plastic, or other transparent material with opposite surfaces either or both of which are curved, by means of which light rays are refracted so that they converge or diverge to form an image.
2. n. A combination of two or more such pieces, sometimes with other optical devices such as prisms, used to form an image for viewing or photographing. Also called compound lens.
3. n. A device that causes radiation other than light to converge or diverge by an action analogous to that of an optical lens.
4. n. A transparent, biconvex body of the eye between the iris and the vitreous humor that focuses light rays entering through the pupil to form an image on the retina.
5. v. Informal To make a photograph or movie of.

Century Dictionary and Cyclopedia

1. n. A piece of transparent substance bounded by two curved surfaces (usually spherical), or by a curved surface and a plane. The ordinary use of a lens is to cause pencils of rays to converge or diverge systematically after passing through it. Lenses for optical purposes are usually made of glass; acoustic lenses, of carbon dioxid inclosed between two thin membranes; lenses for action upon electrical radiations, of paraffin or pitch, substances which are transparent to electrical rays, though opaque to light. Optical lenses alone are in common use. Ordinary lenses are distinguished into two classes—convex or magnifying lenses, which are thickest in the center, and concave, which are thinnest in the center. Each class has three varieties, as shown in fig. 1. To the first belong D, the double-convex or biconvex; C, the plano-convex; and E, the meniscus. The concave lenses are B, the double-concave or biconcave; A, the plano-concave; and F, the concavo-convex, sometimes improperly called concave meniscus. The line which passes through the centers of curvature of the two surfaces is the axis of the lens, and a point on this axis so taken that every line drawn through it pierces parallel elements of the two surfaces is its optical center. A convex lens converges rays which are parallel to its axis, approximately to a point called its principal focus (F in fig. 2). The distance from the optical center to this focus is the same on both sides of the lens, and depends upon the radii of its curved surfaces and the material of which it is made. Rays diverging from a point beyond the principal focus F on either side of the lens are approximately collected to a “real” focus beyond the principal focus on the other side (see fig. 3); but if the source of light is between the lens and its principal focus, the rays after emergence diverge as if they came from a so-called virtual focus behind the luminous point. The luminous point and its focus are interchangeable, and are called conjugate foci, as, for instance, L. and 1 in fig. 3. (See focus, 1.) A concave lens always renders still more divergent rays emanating from a point, and so forms only virtual foci. If the source of light is an extended surface, then the pencil of rays emanating from each point forms its own focus; and the collection of foci constitutes an image, which is real and inverted if the foci are real, but virtual and erect if they are virtual. The relative sizes of the object and image are sensibly proportional, if the lens is thin, to their respective distances from the optical center; if the lens is thick, the distances must be reckoned from the two so-called principal points of the lens (see principal point, under point), which lie on the axis on each side of the optical center. An image formed by a single lens is never perfectly distinct, on account of the spherical and chromatic aberrations of the lens. (See aberration, 4.) The former is due to the fact that a lens bounded by spherical surfaces converges marginal rays to a point nearer the lens than that in which the central rays meet; the latter, to the fact that rays of different color form thoir foci at different distances, the focal distance for violet rays being (with a glass lens) nearly a seventh part shorter than that for the red rays. The spherical aberration can be corrected by making the surfaces of forms other than spherical, or by combining two or more lenses properly proportioned; the chromatic aberration, only by combining two or more convex and concave lenses of different materials, usually a convex of crown-glass with a concave of flint-glass.
2. n. In anatomy, in the eye, a double-convex body placed in the axis of vision behind the iris between the aqueous humor and the vitreous humor, serving to focus rays of light upon the retina; the crystalline lens. See first cut under eye.
3. n. Figuratively, photography, from the use of lenses in that art.
4. n. A genus of leguminous plants of the tribe Vicieæ. It is distinguished from Vicia by having but two ovules instead of many, as is generally the case in Vicia. The 8 species enumerated by some are generally reduced to 2, which are low erector half-climbing herbs with pinnate leaves and small single or racemose pale-blue flowers, natives of the Mediterranean region and eastern Asia. One species, Lens esculenta, the seeds of which are called lentils, is probably one of the oldest of plants cultivated by man for food. See lentil.
5. n. A genus of dicotyledonous plants of the family Mimosaceæ. There are about 14 species, of which the best-known is L. phaseoloides, the match-box bean or simitar-pod. See Entada, gogo, sea-bean, 1, and simitar-pod.
6. n. In entomology: The crystalline lens or cone.
7. n. One of the facets of the compound eye.
8. n. In geology, a body of ore having a lenticular shape. This type is specially common in metamorphic rocks, such as schists or slates, and is very freqnent with magnetic and specular iron ores, pyrites, and some gold-quartz veins. Lenses of magnetite or pyrites often overlap like shingles.
9. n. A surface-condenser made of two round, dished plates bolted together, resembling in form a double-convex lens.

Wiktionary

1. n. An object, usually made of glass, that focuses or defocuses the light that passes through it.
2. n. A device which focuses or defocuses electron beams.
3. n. geometry A convex shape bounded by two circular arcs, joined at their endpoints, the corresponding concave shape being a lune.
4. n. biology A genus of the legume family; its bean.
5. n. anatomy The transparent crystalline structure in the eye.
6. n. by extension, figuratively A way of looking, literally or figuratively, at something.
7. v. transitive To film, shoot.
8. v. geology To become thinner towards the edges.

GNU Webster's 1913

1. n. (Opt.) A piece of glass, or other transparent substance, ground with two opposite regular surfaces, either both curved, or one curved and the other plane, and commonly used, either singly or combined, in optical instruments, for changing the direction of rays of light, and thus magnifying objects, or otherwise modifying vision. In practice, the curved surfaces are usually spherical, though rarely cylindrical, or of some other figure.

WordNet 3.0

1. n. biconvex transparent body situated behind the iris in the eye; its role (along with the cornea) is to focuses light on the retina
2. n. (metaphor) a channel through which something can be seen or understood
3. n. a transparent optical device used to converge or diverge transmitted light and to form images
4. n. genus of small erect or climbing herbs with pinnate leaves and small inconspicuous white flowers and small flattened pods: lentils
5. n. electronic equipment that uses a magnetic or electric field in order to focus a beam of electrons