If a spherical lens has a power of, −0.25 D, the focal length of this lens will be :(a) −4 cm (b) −400 mm (c) −4 m (d) −40 cm


(c) -4 m


Explanation

Given:

Power of the lens, $P$ = $-$0.25 D

To find:  Focal length of the lens, $f$.

Solution:

Power of a lens is given by-

$P=\frac {1}{f}$

Substituting the given values we get-

$-0.25=\frac {1}{f}$

$f=\frac {1}{-0.25}$

$f=-\frac{100}{25}$

$f=-4m$

Thus, the focal length of the lens is 4 m. The minus sign $(-)$ indicates that the lens is diverging in nature means it is a concave lens.

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Updated on: 10-Oct-2022

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