Lens

Lens is a piece of transparent material that has at least one curved surface. Lenses refract (bend) light rays and in doing so can form images of an object. The images may be larger, smaller, or the same size as the object itself. Scientists sometimes use lenses to concentrate or spread a beam of light.

Lenses are an important part of eyes. They enable the eyes to form sharp images of near and distant objects. Lenses in the form of glasses and contact lenses are used to correct imperfections in eyesight. They are also a key part of binoculars, cameras, microscopes, projectors, telescopes, and many other devices.

Artificial lenses are made of various transparent materials, such as glass, plastic, or crystals. Quartz crystals are used to refract ultraviolet light, which has a very short wavelength.

There are two kinds of simple lenses, converging lenses and diverging lenses. They differ in the way they are curved and therefore in the manner in which they refract light. Compound lenses consist of two or more simple lenses fitted together.

Converging lenses, also called positive lenses or convex lenses, are thicker in the middle than at the edges. When parallel rays of light pass through this type of lens, they are bent inward and meet at a point called the focus. The distance from the center of the lens to the focus is known as the focal length.

The size, position, and type of image produced by a converging lens vary according to the distance of the object from the lens. If an object is more than one focal length from the lens, an inverted real image of it is formed on the opposite side of the lens. Light rays from the object pass through a real image and can be focused on a screen. When an object is located a distance of two focal lengths from a converging lens, the image is the same size as the object and is located a distance of two focal lengths on the opposite side of the lens. A smaller image of the object can be obtained by moving the object more than two focal lengths from the lens. A larger image can be produced by placing the object between one and two focal lengths from the lens.

If an object is less than one focal length from the lens, no real image can be formed. Instead, a magnified virtual image is formed behind the object and is right side up. Light rays from the object do not pass through a virtual image, and such an image cannot be focused on a screen.

Diverging lenses are also called negative lenses or concave lenses. Diverging lenses are thicker at the edges than they are at the center. Light rays passing through a diverging lens are bent outward. Diverging lenses form only virtual images. The images are always right side up and reduced in size.

Compound lenses. Simple lenses generally produce aberrated (imperfect) images. These imperfections in image formation can be reduced through the use of compound lenses. For example, diverging lenses are often used to correct aberrations produced by converging lenses. See Aberration.

Contributor: Sandra M. Faber, Ph.D., Prof. of Astronomy, Lick Observatory, Univ. of California, Santa Cruz.

Lens

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