The optician's prescription for a spectacle lens is marked +0.5 D. What is the :(a) nature of spectacle lens?(b) focal length of spectacle lens?
(a) Since the power of the spectacle has a plus sign $(+)$, it implies that it is a converging lens or convex lens.
(b) Given:
Power of the lens, $P$ = $+$0.5 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-
$0.5=\frac {1}{f}$
$f=\frac {1}{0.5}$
$f=\frac {10}{5}$
$f=+2m$
Thus, the focal length of the lens $f$ is 2 m.
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