The earliest lenses were made of circular pieces of rock crystal or semiprecious stone, such as beryl and quartz, which were ground and polished so that they produced a magnified image when looked through. The oldest known lens artifact was one made of rock crystal dating from around 640 B.C.E. and excavated in Nineveh, near the modern city of Mosul, Iraq. The most common form was circular and thicker in the middle than around the edge, and having both its front and back surfaces the same shape.
The modern convex lens developed from the ancient Greek burning glass. Here a spherical vase of water would be used to concentrate the rays of the sun onto a small area, which heated up. The heat was used to ignite fires in temples or to cauterize wounds.
The Iraqi mathematician and optics engineer Ibn Sahl (c 940-1000) wrote the treatise On Burning Mirrors and Lenses (984) in which he set out his understanding of how curved mirrors and lenses bend and focus light, using what is now known as Snell's law to calculate the shape of lenses. But the Iraqi Ibn al- Haytham (965-1039), also known as Alhazen, is regarded as "the father of optics" for his treatise, the Book of Optics, (1011-1021), in which he proved that rays of light travel in straight lines, explained how the lens in the human eye forms an image on the retina, and described experiments with a pinhole camera.
In the thirteenth century convex lenses were used in spectacles to correct farsightedness. The use of concave lenses, which disperse the light as opposed to concentrating it, to correct for nearsightedness, came in the early fifteenth century.
