A lens is an optically effective element with two light-refracting surfaces. Of which a surface is usually curved as concave or convex. The most important characteristic of a lens or a so-called lens system is the optical image. The critical size of a lens is the focal length the unit is specified in meters, depending on the intended use etc. Thus the distance from the focal plane or the focal point of the lens is defined. The simplest lenses are optically active at both spherical surfaces (surface sections of a sphere). From a historical perspective it is assumed that the first glass lenses were used in the field of optics. They were used as lenses to correct short sight or long sight. Later, the first binoculars and also microscopes were developed. Materials which are suitable for the production of lenses must be made of transparent materials. In addition to glass are also some plastics suitable, such as Polycarbonate. The name of the lenses is defined by the curvature of their surfaces. Each of these surfaces can be concave, convex or flat (plan). Convex lenses have curved surfaces outwards, concave lenses, however, have inner curved surface. With lenses having a flat surface no curvature is present. A very important characteristic of a lens is the principle of reversal of the light path. The focal length is dependent on the curvature radii of the two lens surfaces. There are many different types of lenses. We want to enumerate only a few at this moment.
Glass is a material with a long history. By about 5000 BCE, natural glass called obsidian, was used as a working material. Blades, wedges and scrapers were fashioned and used to make other objects for daily use.
Glass was fashioned into containers in Egypt from approx. 1450 BCE. The Egyptians were the ones who first discovered how to make glass. From that point on, glass making developed in leaps and bounds. In the Roman Empire, villas for the upper-classes were outfitted with window glazing. This was made possible by the invention of the glass blowing pipe in approx. 200 BCE.