|
A lens is a transmissive optical device that affects the focus of a light beam through refraction. A simple lens consists of a single piece of material, while a compound lens consists of several simple lenses (''elements''), usually along a common axis. Lenses are made from transparent materials such as glass, ground and polished to a desired shape. A lens can focus light to form an image, unlike a prism, which refracts light without focusing. Devices that similarly refract radiation other than visible light are also called lenses, such as microwave lenses or acoustic lenses. ==History== The word ''lens'' comes from the Latin name of the lentil, because a double-convex lens is lentil-shaped. The genus of the lentil plant is ''Lens'', and the most commonly eaten species is ''Lens culinaris''. The lentil plant also gives its name to a geometric figure. The variant spelling ''lense'' is sometimes seen. While it is listed as an alternative spelling in some dictionaries, most mainstream dictionaries do not list it as acceptable.〔 Reports "lense" as listed in some dictionaries, but not generally considered acceptable.〕〔 Lists "lense" as an acceptable alternate spelling.〕 The oldest lens artifact is the Nimrud lens, dating back 2700 years (7th century B.C.) to ancient Assyria. David Brewster proposed that it may have been used as a magnifying glass, or as a burning-glass to start fires by concentrating sunlight.〔 Another early reference to magnification dates back to ancient Egyptian hieroglyphs in the 8th century BC, which depict "simple glass meniscal lenses". The earliest written records of lenses date to Ancient Greece, with Aristophanes' play ''The Clouds'' (424 BC) mentioning a burning-glass (a biconvex lens used to focus the sun's rays to produce fire). Some scholars argue that the archeological evidence indicates that there was widespread use of lenses in antiquity, spanning several millennia. Such lenses were used by artisans for fine work, and for authenticating seal impressions. The writings of Pliny the Elder (23–79) show that burning-glasses were known to the Roman Empire,〔Pliny the Elder, ''The Natural History'' (trans. John Bostock) (Book XXXVII, Chap. 10 ).〕 and mentions what is arguably the earliest written reference to a corrective lens: 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, ''The Natural History'' (trans. John Bostock) (Book XXXVII, Chap. 16 )〕 Both Pliny and Seneca the Younger (3 BC–65) described the magnifying effect of a glass globe filled with water. Excavations at the Viking harbour town of Fröjel, Gotland, Sweden discovered in 1999 the rock crystal Visby lenses, produced by turning on pole lathes at Fröjel in the 11th to 12th century, with an imaging quality comparable to that of 1950s aspheric lenses. The Viking lenses were capable of concentrating enough sunlight to ignite fires.〔 〕 Between the 11th and 13th century "reading stones" were invented. Often used by monks to assist in illuminating manuscripts, these were primitive plano-convex lenses initially made by cutting a glass sphere in half. As the stones were experimented with, it was slowly understood that shallower lenses magnified more effectively. Lenses came into widespread use in Europe with the invention of spectacles, probably in Italy in the 1280s. This was the start of the optical industry of grinding and polishing lenses for spectacles, first in Venice and Florence in the thirteenth century,〔Al Van Helden. ('''The Galileo Project > Science > The Telescope ). Galileo.rice.edu. Retrieved on 6 June 2012.〕 and later in the spectacle-making centres in both the Netherlands and Germany. 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). 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.〔(Microscopes: Time Line ), Nobel Foundation. Retrieved 3 April 2009〕 With the invention of the telescope and microscope there was a great deal of experimentation with lens shapes in the 17th and early 18th centuries trying to correct chromatic errors seen in lenses. Opticians tried to construct lenses of varying forms of curvature, wrongly assuming errors arose from defects in the spherical figure of their surfaces.〔This paragraph is adapted from the 1888 edition of the Encyclopædia Britannica.〕 Optical theory on refraction and experimentation was showing no single-element lens could bring all colours to a focus. This led to the invention of the compound achromatic lens by Chester Moore Hall in England in 1733, an invention also claimed by fellow Englishman John Dollond in a 1758 patent. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Lens (optics)」の詳細全文を読む スポンサード リンク
|