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what is called lens? How many type of lens are there.

A lens is a piece of some transparent material which is bound by two curved surfaces or by one curved surface and the other plane surface.

Concave or diverging lens :It is thicker at the center than at the edges.It diverges a beam of light on refraction through it.It has a virtual focus.
Types of concave lenses:

(a) Double concave or biconcave lens : Both the surfaces are concave.It is simply called a concave lens. 
(b)Planoconcave lens : One surface is concave and the other is plane.
(c) Convexoconcave : One surface is concave and the other is convex.

Convex or converging lens: It is thicker at the center than at the edges.It converges a parallel beam of light on refraction through it.It has a real focus.

Types of convex lenses :
(a) Double convex or biconvex lens : Both the surfaces are convex.It is simply called a convex lens.
(b)Planoconvex lens : One surface is convex and the other is plane.
(c)Concavoconvex : One surface is convex and the other is concave.


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