Lens, a curved, transparent body (usually of glass) that refracts (bends) light rays. Various types of lenses have different functions. The lens of an eye or camera focuses light rays. In telescopes, microscopes, and magnifying glasses, lenses enlarge the apparent size of objects. A lens in a reducing glass reduces apparent size. In a motion-picture or slide projector, lenses project images so that they will appear on a screen. Some lenses are used to bend light rays in such a way as to correct distortions; examples are the internal elements of a compound camera lens and the lenses used in eyeglasses.
The bending of light rays is explained in the article Refraction. A lens refracts light rays in such a way that one of three things will occur:
- The rays will come together at a point. This occurs when a lens is being used as a burning glass, a device used by the ancient Greeks, Romans, and Chinese to start fires.
- The rays will produce an image. In such instruments as a telescope, the image is of the object producing the rays (such as a star) or of an object reflecting light rays (such as a distant tree). In a motion-picture or slide projector, the image is created by light rays passing through motion-picture film or a slide.
- The rays will move in parallel lines (as in a searchlight) or in diverging lines (as in a reducing glass).
There are two principal kinds of lenses: (1) converging, or convex; and (2) diverging, or concave. A converging lens is always thicker in the center than at the edges. It bends light rays in such a way that they will come together at a point called the focus. A diverging lens is always thicker at the edges than in the center. It makes light rays diverge, or move away from each other. Such a lens has no real focus but has an imaginary one called the virtual focus. The various kinds of converging and diverging lenses are shown in the top diagram on the facing page. (This diagram, like the others in this article, shows lenses as seen from the side.)
A lens may produce a real image or a virtual image. A real image may be projected on a screen or on film in a camera. It is always inverted (upside down) and may be larger than, smaller than, or the same size as the object. Only converging lenses can produce real images. A virtual image can be seen by looking through a lens but cannot be projected. It is always erect (right side up) and may be smaller or larger than the object being viewed.
A single lens will produce a form of distortion called aberration. Aberration can be greatly reduced by using several lenses of different shapes in combination. The main use of diverging lenses is in such a combination.
The refraction of light is always the same under identical circumstances, allowing physicists to draw up mathematical laws of optics. These laws are used in determining the shape of a lens for a particular purpose. The shape is computed mathematically and is expressed by a formula that guides the lensmaker in his or her work.
The glass used for a lens is of the highest quality. It is first molded into blanks, which are disks about the size of the finished lenses. A lens is formed by grinding and polishing a blank into shape. Grinding operations are performed by revolving dish-shaped devices coated with abrasives. The first grinding, with a carborundum abrasive, gives the lens its general shape. Later grindings, with finer and finer abrasives, give it its final shape. The lens is then polished with rouge (fine ferrous oxide) and cut to the proper size.
The early history of lenses is unknown. In 1845 an archeologist uncovered in what is now Iraq an ancient rock crystal ground to form a small convex lens, but there is no evidence that lenses were widely known or used in ancient times. An early investigation of the principles of lenses was made in the 11th century by Alhazen, a Persian physicist. Spectacles with convex lenses were in common use both in Europe and in China as early as the 13th century.
Zacharias Janssen, a Dutch optician, is credited with combining lenses to make a compound microscope about 1590. Galileo improved the telescope in 1609. The art of designing and manufacturing.lenses has progressed steadily since that time.