# Lens

Lenses are carefully shaped pieces of glass, plastic, or other transparent material. They are designed to manipulate light rays to create particular kinds of images. For example, the lenses in a telescope are designed to produce an enlarged view of a faraway object. Other common form of lenses are those found in eyeglasses, cameras, and microscopes.

## Pioneers in lens development

Italian scientist Galileo Galilei (15641642) and Dutch scientist Antoni van Leeuwenhoek (16321723) were among the first to use lenses extensively in scientific research. Other scientistsFrench mathematician René Descartes (pronounced ren-AY day-KART; 15961650) and English scientist Isaac Newton (16421727), among othersdedicated most of their lives to improving lens designs. Despite the amount of time it has been in existence, the lens remains one of the simplest and most useful optical tools available.

## How lenses work

Lenses work on two principles: that light always travels in straight lines, and that it travels more slowly through glass or plastic than it does through air.

Light bends when it exits one substance (the air) and enters another (a lens). It bends again as it leaves the lens. The amount of bending depends greatly upon how much the lens is curved. All lenses have at least one curved surface, and most have two. There are two kinds of lenses, classified by how they are curved. Convex lenses (also called converging or positive) are thick in the middle and thin along the edges. Concave lenses (also called diverging or negative) are thin in

the middle and thick along the edges. Each design bends and affects light differently.

## Convex lenses

A convex lens bends light toward a central point (see Figure 1a). The farther from the center of the lens a beam of light strikes, the more the resulting light (f ) is bent. Assuming an object is more than one focal length (a specific distance determined by the construction of the lens) away from the lens, the image viewed through a convex lens is always upside down. This is called a real image, and it can be projected upon a screen. The real image can be smaller or larger than the original object, also depending upon its distance from the lens.

Convex lenses magnify or enlarge objects. This type of lens is used in microscopes, telescopes, and binoculars.

## Concave lenses

Concave lenses bend light away from a central axis (see Figure 1b). Similar to a convex lens, the light that strikes near the edge of the concave lens is bent more sharply away from the central axis (f ). The image seen through a concave lens is called a virtual image. It is always right side up and cannot be projected. The virtual image is always smaller than the original, no matter what its distance from the lens.

Individual lenses cannot form sharp, flawless images over a wide field, and the images are always accompanied by distortion and color aberrations. Thus, most optical devices use systems of lenses that often assemble convex and concave lenses in precise combinations to minimize distortion or produce various effects.

Certain lenses, called plano-concave and plano-convex, are curved on only one side. Optical correction lenses, such as those used in eyeglasses, are ground with one side concave and one convex. Convexo-concave lenses aid patients who are nearsighted, while farsighted patients require concavo-convex lenses.

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## lens

lens / lenz/ • n. a piece of glass or other transparent substance with curved sides for concentrating or dispersing light rays, used singly (as in a magnifying glass) or with other lenses (as in a telescope). ∎  the light-gathering device of a camera, typically containing a group of compound lenses. ∎  Physics an object or device that focuses or otherwise modifies the direction of movement of light, sound, electrons, etc. ∎  Anat. short for crystalline lens. ∎ short for contact lens. DERIVATIVES: lensed adj. lens·less adj. ORIGIN: late 17th cent.: from Latin, ‘lentil’ (because of the similarity in shape).

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## lens

lens A transparent biconvex structure in the eyes or analogous organs of many animals, responsible for directing light onto light-sensitive cells. In vertebrates it is a flexible structure centred behind the iris and attached by suspensory ligaments to the ciliary body. In terrestrial species its main function is to focus images onto the retina. To focus on near objects, the circular muscles in the ciliary body contract and the lens becomes more convex; contraction of the radial muscles in the ciliary body flattens the lens for focusing on distant objects (see also accommodation).

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## lens

lens (lenz) n.
1. (in anatomy) the transparent crystalline structure situated behind the pupil of the eye. It helps to refract incoming light and focus it onto the retina. See also accommodation.

2. (in optics) a piece of glass or other material shaped to refract rays of light in a particular direction. Lenses are worn to correct faulty eyesight. l. implant a plastic lens to replace a natural lens that has been removed because of cataract. See also bifocal lens, contact lenses, multifocal lens.

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## Lens (city, France)

Lens (läNs), city (1990 pop. 35,278), Pas-de-Calais dept., N France. During the 19th and 20th cent. it was one of the most important coal centers in N France, but its mines are now closed. Much of the city's manufacturing has been replaced by service-oriented industries. The victory there (1648) of the French under Louis II de Condé was the last important battle of the Thirty Years War. Lens was occupied and devastated by the Germans in both world wars. A branch of the Louvre museum opened here in 2012.

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## lens (in optics)

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.

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## lens

lens Piece of transparent glass, plastic, quartz, or organic matter, bounded by two surfaces (usually both spherical) that changes the direction of a light beam by refraction (bending the wave). A convex lens bends light rays towards the lens axis. A concave lens bends rays away from the axis. The optical image may be right side-up or inverted, real or virtual, and magnified or reduced in size.

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## lens

lens XVII. — L. lens LENTIL; so called on account of its shape.

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## lens

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