The earliest known lenses were made from polished crystal, often quartz, and have been dated as early as 750 BC. The focal length of a lens is the measure of the power of the lens. Lenses work on the principle of refraction and can be of Convex, Concave as well as of Compound type. They have several applications in Binoculars, Telescopes, Electron Microscopes, Movie Projectors, Cameras and Data Storage Media. It’s interesting that Lighthouses use amazing Fresnel lenses. Until the advent of plastics, they were manufactured with grinding solid glass which have been replaced with more refined optical class.