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The power of a concave lens is 10 D and that of a convex lens is 6 D. When these two lenses are placed in contact with each other, the power of their combination will be :
(a) +16 D (b) +4 D (c) −16 D (d) −4 D
(a) -4 D
Explanation
Given:
Power of the concave lens, $P_1$ = $-$10 D (power of concave lens is always taken negative)
Power of the convex lens, $P_2$ = $+$10 D (power of convex lens is always taken positive)
To find: Combined power of the two lenses, $P$.
Solution:
The power of the combination of lenses is given as-
$P=P_1+P_2$
Substituting the given value, we get-
$P=6D+(-10D)$
 $P=-4D$
Hence, the combined power of the two lenses, $P$ is -4D.
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