**Spherical Lens**

A transparent material bound by two surface, of which one or both surfaces are spherical, forms a lens.

**Convex 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.

**Concave Lens**

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 C_{1} and C_{2}.

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 OF_{2} and OF_{1} 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**