Contact lens is a thin lens placed on the eye surface that can be weared for vision correction, esthetical or treatment purposes. Compared to eyeglasses contact lenses serve a better peripheral vision and the fact that they do not get dirty or dusty when weared is an advantage as well.
The idea of a contact lens was lounched by Leonardo da Vinci by pacing a hemisphere filled with water on the eye surface in 1508. This was developed by Descartes (1636) and Young (1801) later. First succesful contact lenses of blown glasses were produced by German ophthalmologes at the end of 1880s.
Soft and rigid contact lenses are the main types. The usage range of rigid lenses are narrowed to corneal disfiguraitons like keratoconus. Lens materials have been evolved considerably for the aspects of oxygen diffusability, water ingredient, thiness and lubricity of the surface. Myopia, hypermetropia nad astigmatism can be corrected. Multifocal contact lenses with the near portion on the lower side are available.
Hygene is a principle in the contact lens usage, patients should get a training about the right usage from an ophthalmologist or authorised people.
Usage of glasses as a magnifier for reading dates back to 1000 years. Sight correcting glasses weared in front of eyes for a better vision were mentioned in Marco Polo's travelogue in 1200s. Kepler defined in 1604 first that hypermetropis can be corrected via convex lens and myopia via concave lens.
Eye glasses are corrective lenses weared in front of eyes for correcting refractive errors. Myopic eyes have focal points anterior to the retina, that can be moved back onto the retina via concave lenses. Hypermetropic nad presbiopic eyes have focal points posterior to the retina, that can be moved forward onto the retina via convex lenses. Astigmatism is corrected via cylindrical lenses that are set to the main meridians so light beams focused in two meridians can be merged on retina.
Besides concavity and convexity, namely refractive powers, lenses have prizmatic effects as well. That means a concave lens tends to accumulate the image to the middle portion of the lens and a convex lens tends to spread the image lightly to the edges. With these features lenses serve the purpose of correcting some squint types.
Reading glasses can be mounted to the lower portion of far glasses instead of seperate usage. Like bifocal types eyeglasses can be multifocal and can have different focal zone with various settings.
As lightness keeps the emphasis lenses of an eyeglass are organic plastic and can be thinned. They are covered with various filters against sunshine, dust, water or dirt.