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.