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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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