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# Refraction at Spherical Surfaces and by Lenses - Power of a Lens

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## Power of lens:

A convex lens with short focal length converges the light rays with greater degree nearer to principal focus and a concave lens with short focal length diverges the light rays with greater degree nearer to principal focus.

The degree of divergence or convergence of ray of light by a lens is expressed in terms of the power of lens. Degree of convergence and divergence depends upon the focal length of a lens. The power of a lens is denoted by ‘P’. The power of a lens is reciprocal of the focal length.

Power = 1/("Focal length f")

or, P = 1/"f"

The SI unit of Power of lens is dioptre and it is denoted by ‘D’.

Power of a lens is expressed in dioptre when the focal length is expressed in metre. Thus, a lens having 1 metre of focal length has power equal to 1 dipotre.

Therefore, 1 D = 1 m−1

A convex lens has power in positive and a concave lens has power in negative.

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Ray Optics part 34 (Power of lens) [00:04:13]
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