Question

define what is lens and how it is useful for us

Answer 1

A lens is a transmissive optical device that focuses or disperses a light beam by means of refraction.

uses

Lenses are used as prosthetics for the correction of visual impairments such as myopia, hypermetropia, presbyopia, and astigmatism. (See corrective lens, contact lens, eyeglasses.) Most lenses used for other purposes have strict axial symmetry; eyeglass lenses are only approximately symmetric. They are usually shaped to fit in a roughly oval, not circular, frame; the optical centres are placed over the eyeballs; their curvature may not be axially symmetric to correct for astigmatism. Sunglasses' lenses are designed to attenuate light; sunglass lenses that also correct visual impairments can be custom made.

Other uses are in imaging systems such as monoculars, binoculars, telescopes, microscopes, cameras and projectors. Some of these instruments produce a virtual image when applied to the human eye; others produce a real image that can be captured on photographic film or an optical sensor, or can be viewed on a screen. In these devices lenses are sometimes paired up with curved mirrors to make a catadioptric system where the lens's spherical aberration corrects the opposite aberration in the mirror

Answer 2

***Piece of transparent substance, usually glass, having two opposite surfaces either both curved or one curved and one plane,used in an optical device in changing the convergence of light rays, as for magnification, or in correcting defects of vision is termed as Lens.

An optical lens is generally made up of two spherical surfaces.

=>If those surfaces are bent outwards, the lens is called a biconvex lens or simply convex lens. 
=>If one of the surfaces is flat and the other convex, then it is called a plano-convex lens.
=>There is another type of lens known as concave lens.
¤ Concave Lens will always produce diminished, upright and virtual image of the object in front of it.
-But the nature of the image produced by a these lens depends upon the position of the object.
-It can form both real and virtual images depending on where the light is coming from.


Biconvex lens

1.Biconvex lens are the simple lenses which comprises of two convex surfaces in spherical form, generally having the same kind of radius of curvature.
* These are also called as convex-convex lens.

2. In a biconvex lens or a Plano-convex lens, a collimated or accurately parallel beam of light passes through the lens and converges at a spot or focus coming behind the lens.
*In such case, this lens is known as a converging or positive lens. 
*The distance coming from the lens up to the spot is called as the focal length of lens.

3. Since there is a curvature on both the sides of lens, there will be around two focal points and also two centers.
The line that is drawn through middle of the biconvex lens is known as the principal axis. 

4. Human eye lens is an best example of an Biconvex lens

5.Uses:

a) Biconvex lens are used as a magnifying or condensing lenses.
b) Also used as objectives or magnifiers
c) These are utilized in many of imaging systems such as the telescopes, monocular, microscopes, binoculars, cameras, projectors, etc.
d) These are used in producing the virtual image in case of a human eye and a real image in case of the photographic films or an optical sensor
e) These lenses are also used as a burning-glass.
f) These are used in the applications of image relays and also in various kinds of industries
g) used to correct Hypermetropia or long-sightedness.