If the lens is biconvex or plano-convex, a collimated beam of light passing through the lens converges to a spot (a focus) behind the lens. It is this type of lens that is most commonly used in corrective lenses. A lens with one convex and one concave side is convex-concave or meniscus. If one of the surfaces is flat, the lens is plano-convex or plano-concave depending on the curvature of the other surface. A lens with two concave surfaces is biconcave (or just concave). If both surfaces have the same radius of curvature, the lens is equiconvex. A lens is biconvex (or double convex, or just convex) if both surfaces are convex. Lenses are classified by the curvature of the two optical surfaces. The more complicated shapes allow such lenses to form images with less aberration than standard simple lenses, but they are more difficult and expensive to produce. These are lenses where one or both surfaces have a shape that is neither spherical nor cylindrical. An example is eyeglass lenses that are used to correct astigmatism in someone's eye. They have a different focal power in different meridians. Toric or sphero-cylindrical lenses have surfaces with two different radii of curvature in two orthogonal planes. The lens axis may then not pass through the physical centre of the lens. Lenses may be cut or ground after manufacturing to give them a different shape or size. Typically the lens axis passes through the physical centre of the lens, because of the way they are manufactured. The line joining the centres of the spheres making up the lens surfaces is called the axis of the lens. Each surface can be convex (bulging outwards from the lens), concave (depressed into the lens), or planar (flat). Most lenses are spherical lenses: their two surfaces are parts of the surfaces of spheres. This section needs expansion with: history after 1758. The practical development and experimentation with lenses led to the invention of the compound optical microscope around 1595, and the refracting telescope in 1608, both of which appeared in the spectacle-making centres in the Netherlands. Spectacle makers created improved types of lenses for the correction of vision based more on empirical knowledge gained from observing the effects of the lenses (probably without the knowledge of the rudimentary optical theory of the day). This was the start of the optical industry of grinding and polishing lenses for spectacles, first in Venice and Florence in the late 13th century, and later in the spectacle-making centres in both the Netherlands and Germany.
Spectacles were invented as an improvement of the "reading stones" of the high medieval period in Northern Italy in the second half of the 13th century. The medieval (11th or 12th century) rock cystal Visby lenses may or may not have been intended for use as burning glasses. These were primitive plano-convex lenses initially made by cutting a glass sphere in half.
Between the 11th and 13th century " reading stones" were invented. The Arabic translation of Ptolemy's Optics became available in Latin translation in the 12th century ( Eugenius of Palermo 1154). The book was, however, received, by medieval scholars in the Islamic world, and commented upon by Ibn Sahl (10th century), who was in turn improved upon by Alhazen ( Book of Optics, 11th century). Ptolemy (2nd century) wrote a book on Optics, which however survives only in the Latin translation of an incomplete and very poor Arabic translation. Both Pliny and Seneca the Younger (3 BC–65 AD) described the magnifying effect of a glass globe filled with water.
Pliny also has the earliest known reference to the use of a corrective lens when he mentions that Nero was said to watch the gladiatorial games using an emerald (presumably concave to correct for nearsightedness, though the reference is vague). Pliny the Elder (1st century) confirms that burning-glasses were known in the Roman period. The oldest certain reference to the use of lenses is from Aristophanes' play The Clouds (424 BC) mentioning a burning-glass. Others have suggested that certain Egyptian hieroglyphs depict "simple glass meniscal lenses". The so-called Nimrud lens is a rock crystal artifact dated to the 7th century BC which may or may not have been used as a magnifying glass, or a burning glass. Some scholars argue that the archeological evidence indicates that there was widespread use of lenses in antiquity, spanning several millennia. The lentil plant also gives its name to a geometric figure. The word lens comes from lēns, the Latin name of the lentil, because a double-convex lens is lentil-shaped. See also: History of optics and Camera lens
0 kommentar(er)
