A transparent material bound by two surface, of which one or both surfaces are spherical, forms a lens.
A lens may have two spherical surfaces, bulging outwards, is called double convex lens (or simply convex lens.
It is also known as converging lens because it converges the light.
A lens bounded by two spherical surfaces, curved inwards is known as double concave lens (or simply concave lens)
It is also known as diverging lens because it diverges the light.
Few Basic Terms Related to Spherical Lens.
1. Centre of curvature : A lens, either a convex lens or a concave lens is a combination of two spherical surfaces. Each of these surfaces form a part of sphere. The centre of these two spheres are called centre of curvature represented by C1 and C2.
2. Principal axis : Imaginary straight line passing through the two centres of curvature
3. Optical Centre : The central point of lens is its optical centre (O). A ray of light, when passes through ‘O’ it remains undeviated i.e. it goes straight.
4. Aperture : The effective diameter of the circular outline of a spherical lens.
5. Focus of lens : Beam of light parallel is principal axis, after refraction from
1. Convex lens, converge to the point on principal axis, denoted by F, known as Principal focus
2. Concave lens, appear to diverge from a point on the principal axis, known as principal focus.
The distance OF2 and OF1 is called as focal length
Tips for drawing Ray diagram
(a) After refraction, a ray parallel to principal axis will pass through F.
(b) A ray passes through F, after refraction will emerge parallel to principal axis.
(c) A ray passes through optical centre ‘O’, passes without any deviation.
Image formation by concave lens