What is a lens?

Question

I would like to know all about lens.E.g; focal length,principal axis,magnification,working principles of lens and all the diagrams and formulars with derivations.

Answer 1

http://en.wikipedia.org/wiki/Lens_(optics) is a good place to start and learn.

http://optical-components.globalspec.com/LearnMore/Optical_Components_Optics/Optical_Components/Optical_Lenses

is anotherr good link. Both came up on top when I typed Optical Lenses in Google.

Basically lenses (optical lenses) are devices that refract light (bending) and thus help in fomation of images. Used in telescopes and microscopes, vision correcting glasses etc.

Answer 2

A lens is a transparent piece of glass bounded by two curved surfaces on either side.A line perpendicular to the optic axis is called the principal axis, and foal point is on it at the center of the lens.

Answer 3

A lens is a device that causes light to either converge and concentrate or to diverge. It is usually formed from a piece of shaped glass or plastic. Analogous devices used with other types of electromagnetic radiation are also called lenses: for instance, a microwave lens can be made from paraffin wax.
History

The earliest written records of lenses date to Ancient Greece, with Aristophanes' play The Clouds (424 BC) mentioning a burning-glass (a convex lens used to focus the sun's rays to produce fire). The writings of Pliny the Elder (23–79) also show that burning-glasses were known to the Roman Empire[1], and mentions what is possibly the first use of a corrective lens: Nero was said to watch the gladiatorial games using an emerald[2] (presumably concave to correct for myopia, though the reference is vague). Both Pliny and Seneca the Younger (3 BC–65) described the magnifying effect of a glass globe filled with water. The Arabian mathematician Ibn Sahl (c.940–c.1000) used what is now known as Snell's law to calculate the shape of lenses[3], and Ibn al-Haitham (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 artifact 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 that have an imaging quality comparable to that of 1950s aspheric lenses. The Viking lenses concentrate sunlight enough to ignite fires.

Widespread use of lenses did not occur until the invention of spectacles, probably in Italy in the 1280s. Nicholas of Cusa is believed to have been the first to discover the benefits of concave lenses for the treatment of myopia in 1451.

The Abbe sine condition, due to Ernst Abbe (1860s), is a condition that must be fulfilled by a lens or other optical system in order for it to produce sharp images of off-axis as well as on-axis objects. It revolutionized the design of optical instruments such as microscopes, and helped to establish the Carl Zeiss company as a leading supplier of optical instruments.

Lens construction
Image of the city of Seattle as seen through a lens.
Image of the city of Seattle as seen through a lens.

Most lenses are spherical lenses: lenses whose surfaces have spherical curvature, that is, the front and back surfaces of the lens can each be imagined to be part of the surface of a sphere. Types of lenses

Lenses are classified by the curvature of the two optical surfaces. A lens is biconvex (also called double convex, or just convex) if both surfaces are convex, likewise, a lens with two concave surfaces is biconcave (or just concave). 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 one convex and one concave side is 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.

http://en.wikipedia.org/wiki/Lens_%28optics%29

Answer 4

Lens are spherical glasses which do not have a silvered side like spherical mirrors and there are two types,Concave lenses and convex lenses and their use is that concave lenses are used for glasses of the people affected by myopia(cannot see things close to them)and convex lenses' use is that they are used for glasses of the people affected by Hypermetropia(cannot see things far away from them).