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Spherical lenses

Ankit went to an optician and noticed different types of spectacles there. He observed that while the glasses of some spectacles were relatively thicker in the middle, other glasses were thicker on the edge. The glasses of these spectacles are examples of lenses.

A lens is a transparent material bound by two curved surfaces. Lenses are broadly classified into two categories depending on their surfaces.

However, we will discuss only double spherical lenses here.

Convex lens

A convex lens is made by joining two spherical surfaces in such a way that it is thicker at the centre. Its thickness gradually reduces as we move towards the edge.

               

A convex lens has the ability to converge the light rays to a point that are incident on it. Thus, it is called a converging lens.

Concave lens

A concave lens is made by joining two curved surfaces in such a way that it is thinner at the centre. Its thickness gradually increases as we move towards the edge.

                     

A concave lens has the ability to diverge a beam of light rays incident on it. Thus, it is called a diverging lens.

Differences between a spherical mirror and a lens

The following table lists some common differences between spherical mirrors and lenses

Spherical mirror

Spherical lens

Image is formed by reflection of light.

Image is formed by refraction of light.

A spherical mirror has only one focus.

A spherical lens has two foci.

The centre of the spherical mirror is termed as its pole.

The centre of the spherical lens is termed as its optical centre.

The second difference arises due to the fact that a lens has two spherical surfaces (i.e. it can be made from the arc of two spheres of equal radius).Therefore, light is refracted twice before it comes out of the lens.

Terms Associated with Lenses:

Optical centre

Optical centre is a point at the centre of the lens. It always lies inside the lens and not on the surface. It is denoted by ‘O’.

Centre of curvature

It is the centre point of arcs of the two spheres from which the given spherical lens (concave or convex) is made. Since a lens constitutes two spherical surfaces, it has two centers of curvature.

The distance of the optical centre from either of the centre of curvatures is termed as the radius of curvature.

Principal axis

The imaginary straight line joining the two centers of curvature and…

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