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A lens is a device for either concentrating or diverging lens can be made from Paraffin Wax . HISTORY The earliest written records of lenses date to ( 965 – 1038 ) wrote the first major optical treatise which described how the Lens in the human Eye formed an image on the Retina . The oldest lens-artefact however is dated to 640s BC ; a Rock Crystal lens found at excavations in Ninive . Recent excavations at the Viking harbor town of Fröjel , Gotland in Sweden have revealed rock crystal lenses produced at Fröjel in the 11th to 12th century via turning on pole-lathes have been found that have an imaging quality comparable to that of 1950's aspheric lenses. The Viking lenses quite effectively concentrate sunlight enough to ignite fires. Widespread use of lenses did not occur until the invention of Spectacles , probably in Italy in the 1280s . LENS CONSTRUCTION The most common type of lenses are ''spherical lenses'', which are formed from surfaces that have ''spherical curvature'', that is, the front and back surfaces of the lens can be imagined to be part of the surface of two spheres of given radii, ''R''1 and ''R''2, which are called the ''radius of curvature'' of each surface. Sign convention of lens radii ''R''1 and ''R''2 The signs of the lens radii indicate whether the corresponding surfaces are Convex (bulging outwards from the lens) or Concave (depressed into the lens). The sign convention used to represent this varies, but in this article if ''R''1 is positive, the first surface is convex, and if ''R''1 is negative, the surface is concave . If ''R''1 is Infinite , the surface is flat, or has zero curvature, and is said to be ''planar''. The signs are reversed for the back surface of the lens: if ''R''2 is positive, the surface is concave, and if ''R''2 is negative,the surface is convex. Types of lenses The line joining the centres of the spheres making up the lens surfaces is called the ''axis'' of the lens; in almost all cases the lens axis passes through the physical centre of the lens. Lenses are classified by the curvature of these two surfaces. A lens is ''biconvex'' (or just ''convex'') if both surfaces are convex, likewise, a lens with two concave surfaces is ''biconcave'' (or ''concave''). If one of the surfaces is flat, the lens is termed ''plano-convex'' or ''plano-concave'' depending on the curvature of the other surface. A lens with one convex and one concave side is termed ''convex-concave'', and in this case if both curvatures are equal it is a '' Meniscus '' lens. (Sometimes, ''meniscus lens'' can refer to any lens of the convex-concave type.) If the lens is biconvex or plano-convex, a Collimated or parallel beam of light travelling parallel to the lens axis and passing through the lens will be converged (or ''focused'') to a spot on the axis, at a certain distance behind the lens (known as the '' Focal Length ''). In this case, the lens is called a ''positive'' or ''converging'' lens. If the lens is biconcave or plano-concave, a collimated beam of light passing through the lens is diverged (spread); the lens is thus called a ''negative'' or ''diverging'' lens. The beam after passing through the lens appears to be emanating from a particular point on the axis in front of the lens; the distance from this point to the lens is also known as the focal length, although it is negative with respect to the focal length of a converging lens. If the lens is convex-concave, whether it is converging or diverging depends on the relative curvatures of the two surfaces. If the curvatures are equal (a meniscus lens), then the beam is neither converged nor diverged. Lensmaker's equation The value of the focal length for a particular lens can be calculated from the lensmaker's equation: : where :''f'' is the focal length of the lens, :''n'' is the Refractive Index of the lens material, :''nm'' is the refractive index of the Medium the lens is in, :''R''1 is the radius of the lens closest to the light source, :''R''2 is the radius of the lens farthest from the light source, and :''d'' is the thickness of the lens (the distance along the lens axis between the two Surface Vertices ). Refer above to the sign convention associated with ''R''1 and ''R''2. The symbol ''n''' is sometimes used instead of ''nm'' to denote the refractive index of the medium surrounding the lens. Thin lens equation If ''d'' is small compared to ''R''1 and ''R''2, then the '' Thin Lens '' assumption can be made, and ''f'' can be estimated as: : The focal length ''f'' is positive for converging lenses, negative for diverging lenses, and infinite for meniscus lenses. The value 1/''f'' is known as the '' Optical Power '' of the lens, and so meniscus lenses are said to have zero power. Lens power is measured in '' Dioptre s'', which are units equal to inverse meters (m−1). Lenses have the same focal length when light travels from the back to the front as when light goes from the front to the back, although other properties of the lens, such as the Aberrations are not necessarily the same in both directions. IMAGING PROPERTIES As mentioned above, a positive or converging lens will focus a collimated beam travelling along the lens axis to a spot (known as the Focal Point ) at a distance ''f'' from the lens. Conversely, a point source of light placed at the focal point will be converted into a collimated beam by the lens. These two cases are examples of Image formation in lenses. In the former case, an object at an infinite distance (as represented by a collimated beam of light) is focused to an image at the focal point of the lens. In the latter, an object at the focal length distance from the lens is imaged at infinity. The plane perpendicular to the lens axis situated at a distance ''f'' from the lens is called the ''focal plane''. If the distances from the object to the lens and from the lens to the image are ''S''1 and ''S''2 respectively, for a lens of negligible thickness they are found by the thin lens formula: : . What this means is that, if an object is placed at a distance ''S''1 along the axis in front of a positive lens of focal length ''f'', a screen placed at a distance ''S''2 behind the lens will have an image of the object projected onto it, as long as ''S''1 > ''f''. This is the principle behind Photography . The image in this case is known as a '' Real Image ''. Note that if ''S''1 < ''f'', ''S''2 becomes negative, the image is apparently positioned on the same side of the lens as the object. Although this kind of image, known as a '' Virtual Image '', cannot be projected on a screen, an observer looking through the lens will see the image in its apparent calculated position. A Magnifying Glass creates this kind of image. The '' Magnification '' of the lens is given by: : , |