Question

A concave lens has a focal length of 20 cm .calculate is power

Answer 1

Given,

focal length f=20 cm

Power P=?

We know that

P=1/f m

 =100/f cm

 =100/20

 =5D

Therefore, power of lens is 5 Dioptre

Hope it helps

Answer 2

Given that,

Focal length of a concave lens, f = -20 cm = 0.2 m (negative for concave lens.

To find,

Power of a lens.

Solution,

Let P is the power of concave lens. Power of a lens is reciprocal of its focal length. It is given by :

$P=\dfrac{1}{f}\\\\P=\dfrac{1}{-0.2}\\\\P=-5\ D$

So, the power of a concave lens is (-5 D).

Learn more,

Power of a lens

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