Power of lens is a measure of the convergence or divergence, which a lens introduces in the light falling on it.”

It is defined as the tangent of the angle by which it converges or diverges a beam of light falling at unit distant from the optical centre.


\tan \,\delta \, = \,\frac{h}{f};\,\,\,\,\,\,\,\,{\rm{if}}\,h\, = \,1,\,\,\,\,\,\,\,\,\tan \,\delta  = \,\frac{1}{f}\,

or\,\,\,\,\,\,\,\,\,\,\delta \, = \,\frac{1}{f}\,\,\,\,\,\,\,{\rm{for small value of }}\delta .\\
\,\,\,\,\,\,\,\,\,\,\,\,\,P = \delta  = \,\frac{1}{f}

Thus, Power of a lens is the reciprocal of focal length.

When focal length is measured in metre then power is taken in dioptre (D).

For converging lens,f\, = \, + ve, therefore P=+ve

For diverging lens, f\, = \, - ve, therefore P =– ve.

Post Author: E-Physics

Leave a Reply

Your email address will not be published. Required fields are marked *