Question

power of a lens is -2 D. find out the focal length and identify the lens 9) -0.5 m, concave lens b) -0.5 m, convex lens c) + 50 m,concave lens d) -50 m, convex lens​

Answer 1

Given :-

Power of a lens is - 2D

To Find:-

The type of lens and focal length of the lens

Solution :-

Before starting the answer , let's recall how we find the power of lens and identify the lens :)

Consider a spherical lens with focal length " f metres " . Then The power of the lens ( P ) is given by

• ${ \pmb { \green { \bf { P = \dfrac{1}{f}}}}}$

Also , if the focal length is + ve , then the lens is convex . And if focal length is - ve , then the lens is concave .

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Now , in case of this question we can say that ;

$\quad \leadsto \quad \sf \dfrac{1}{f} = - 2$

${ : \implies \quad \sf f = - \dfrac{1}{2}}$

${ : \implies \quad \bf f = - 0.5 \: m }$

Now , as the focal length is - ve . So , the lens is concave .

Henceforth , Option a ) - 0.5 m , concave lens is correct :)