A lens is made of a transparent material bounded by two spherical surfaces.
The surfaces may be both convex, both concave or one convex and one concave etc. For a thin lens, the thickness of the lens is small compared to other dimensions like object distance.
Two basic types are:
(1) Convex lens – thicker in the middle than at the edges.
(2) Concave lens – it is thinner in the middle than at the edges.
As there are two spherical surfaces, there are two centre of curvature C1, C2 and correspondingly two radii of curvature R1, and R2.
Principal axis of a lens: It is a line joining C1 and C2.
Optical centre: It is the centre C of the thin lens, which lies on the principal axis.
Focus: the focus on the side of the (original) source of light is called the first focal point whereas the other is called the second focal point.
If the media on the two sides of a thin lens have same refractive index, then the focal length are equal.