What is meant by power of lens? Explain this. - Vidyadip

Question

What is meant by power of lens? Explain this.

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Answer

Power of lens: The power $P$ of a lens is defined by the angle of incidence through which it reflects a light beam incident at the unit distance from optical centre hence and does convergent or divergent. As shown in fig.
$\tan \delta=\frac{h}{f} \text {; If } h=1 \tan \delta=\frac{1}{f} \text {. }$
or $\quad \delta=\frac{1}{f}$ (for very small value of $\delta$ )
Thus $\quad \underline{P =\frac{1}{f}}$ Here, $f$ is taken in meters.

The SI unit of lens power is diopter (D) : $1 D =1 m^{-1}$. Hence, the power of a lens of focal length 1 m is one diopter. The power of converging lenses is positive and the power of diverging lenses is negative. Thus when an eye doctor prescribes a corrective lens of +2.5 D power, then a convex lens of +40 cm focal length is required. A lens of power -4.0 D means a concave lens of focal length -25 cm .

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