Whenever a ray of light passes through a lens (except when it passes
through the optical center) 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 meters.
The unit of power is dioptre
If there is convex lens of power ID then its focal length is equal 1 meter.
The power of a convex lens is positive and concave lens is negative.
Power of a Lens
The power of a lens is its ability to bend light – the greater the power the greater the refraction of light.
The power of a lens is also related to the focal length by the following formula:
![\[\begin{gathered}
Power\;of\;a\;lens\; = \;\frac{{\text{1}}}{{{\text{focal}}\;{\text{length}}\;{\text{of}}\;{\text{the}}\;{\text{lens}}}} \hfill \\
\hfill \\
P\; = \;\frac{1}{f} \hfill \\
\end{gathered} \]](https://lh3.googleusercontent.com/blogger_img_proxy/AEn0k_tmVFCUlRD2OapGjpkFniB86W5_AYMqMyU59yJRDEBlAnutPOuLsTlTzSjvSK8cRgGbBgkrvLuzIbfxX4nU1gkj6hiJrjJfvUxDgMvaHlLW0bmWEyy7VetQWoUg-Ish1CVdPG5OA7zIT0qGmqqIC9fWE8k=s0-d)
Power of a lens is measured in Dioptres (D).
Focal length is measured in metres (m).
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