Refraction of Light

Whenever a ray of light passes through a lens (except when it passes through the optical centre) it bends. The bending of light rays towards the principal axis is called convergence and bending of light rays away from the principal axis is called divergence. The degree of convergence or divergence of a lens is expressed in terms of its power. A lens of short focal length deviates the rays more while a lens of large focal length deviates the rays less. Thus power of a lens is defined as the reciprocal of its focal length in metres.

Power of a lens =

The unit of power is dioptre

1 dioptre =

1D

If there is convex lens of power 1 D then its focal length is equal 1 metre.

The power of a convex lens is positive and concave lens is negative.

If the focal lengths of the individual lenses f₁ f₂ are f₁, f₂ ....... then the focal length of the equivalent lens is given by

or .

In terms of the power of the lenses P = P₁ + P₂......