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 Much of the city's manufacturing has been replaced by service-oriented industries.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.

lens, in optics

lens, device for forming an image image, in optics, likeness or counterpart of an object produced when rays of light coming from that object are reflected from a mirror or are refracted by a lens .
..... Click the link for more information. of an object by the refraction refraction, in physics, deflection of a wave on passing obliquely from one transparent medium into a second medium in which its speed is different, as the passage of a light ray from air into glass.
..... Click the link for more information. 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 camera obscura [Lat.,=dark chamber] which was literally a dark box—sometimes large enough for the viewer to stand inside—with a small hole, or aperture, in one side. (A lens was not employed for focusing until the Middle Ages.
..... Click the link for more information. , projectors, microscopes microscope, optical instrument used to increase the apparent size of an object.

Simple Microscopes

A magnifying glass, an ordinary double convex lens having a short focal length, is a simple microscope.
..... Click the link for more information. , telescopes telescope, traditionally, a system of lenses, mirrors, or both, used to gather light from a distant object and form an image of it. Traditional optical telescopes, which are the subject of this article, also are used to magnify objects on earth and in astronomy;
..... Click the link for more information. , binoculars binocular, small optical instrument consisting of two similar telescopes mounted on a single frame so that separate images enter each of the viewer's eyes. As with a single telescope, distant objects appear magnified, but the binocular has the additional advantage
..... Click the link for more information. , opera glasses, and eyeglasses eyeglasses or spectacles, instrument or device for aiding and correcting defective sight. Eyeglasses usually consist of a pair of lenses mounted in a frame to hold them in position before the eyes.
..... Click the link for more information. . The lens of the eye eye, organ of vision and light perception. In humans the eye is of the camera type, with an iris diaphragm and variable focusing, or accommodation. Other types of eye are the simple eye, found in many invertebrates, and the compound eye, found in insects and many
..... Click the link for more information. 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 aberration, in optics, condition that causes a blurring and loss of clearness in the images produced by lenses or mirrors. Of the many types of aberration, the two most significant to the lens maker are spherical and chromatic.
..... Click the link for more information. , 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.

lens

Piece of glass or other transparent substance that is used to form an image of an object by converging or diverging rays of light from the object. Because of the curvature of its surface, different rays of light are refracted (see refraction) through different angles. A convex lens causes rays to converge on a single point, the focal point. A concave lens causes rays to diverge as though they are coming from a focal point. Both types cause the rays to form a visual image of the object. The image may be real—inverted and photographable or visible on a screen—or it may be virtual—erect and visible only by looking through the lens.

The glass or plastic elements that focus light onto analog film or a digital sensor in a still or video camera. Lens quality is just as important in digital cameras as it was in the Daguerreotype cameras in the 1800s. See digital camera and kit lens.