Lens: A lens is a piece of a refracting medium bounded by two surfaces, at least one of which is a curved surface. The commonly used lenses are the spherical lenses, which have either both surfaces spherical or one spherical and the other a plane one.
Spherical lenses are of two main types:
(i) Convex or converging lens: It is thicker at the centre than at the edges. It converges a parallel beam of light on refraction through it. It has a real focus.
Fig. Different types of convex lenses
Types of convex 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.
(ii) Concave or diverging lens. It is thinner at the centre than at the edges. It diverges a beam of light on refraction through it. It has a virtual focus.
Fig. Different types of concave lenses
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.
