# Translations of Encyclopedia about Physics

## Lenses

Only based on exact knowledge of the effect and characteristics of lenses and light refraction are we able to understand and construct such optical instruments as cameras, microscopes and photocopiers. The lenses or the entire systems which use them are the foundation of optical instruments.

In optics, a lens is understood as a transparent object made of glass or other, suitable synthetic material. When a light ray falls on a lens, it becomes refracted, meaning, that its direction and speed of travel changes, dependent on the shape of the lens.

Lenses which are thicker in their centre than on their edges are referred to as convex lenses. They connect light rays which fall on it in parallel, for which reason they are referred to as converging lenses. There are three kinds of convex lenses: biconvex (which bulge outwards on both sides), plano-convex (which bulge outwards on one side only, flat on the other) and concave-convex lenses (concave on one side but convex on the other).

Lenses which are thinner in the middle than they are on the edges are referred to as concave lenses. These types of lenses disperse light, for which reason they are referred to as diverging or negative lenses. There are also three kinds of this type of lens: biconcave lenses, which bulge inwards on both sides; plano-concave, where only one side bulges inward and the other side is flat; and convex-concave, where one side is convex and the other concave.

An important characteristic of lenses is their focal distance, which is the distance between the focal point of a lens and the point at which an image of a distance object is projected. The focal point is a particular point located on the optical axis. All light rays travelling in parallel and close to the axis are refracted in such a way that they either converge into the focal point (junction or converging point) or they diverge away from it. There are always two focal points because light can enter from either side of each lens. The focal point can also be referred to as the point where an image of a distant object is projected.

The ancestor of the lens was created long ago but, for optical imaging, it was constructed too non-symetrically and created an image which was too deformed. The first optical lenses were created in the 13th century in Europe and were used to make glasses. As time progressed, the technology to make glasses developed to the point that microscopes and eventually telescopes could be created. The use of lenses in optical lenses has also led to the birth of photography.

When constructing an optical system, we must use a greater number of lenses. For example, cosmic telescopes use this type of complex system and which are able to focus on very distant objects, for which reason they are used to observe heavenly bodies.

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