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The power of a converging lens is 4.5 D and that of a perging lens is 3 D. The power of this combination of lenses placed close together is :(a) +1.5D (b) +7.5D (c) −7.5D (d) −1.5D
(a) 1.5D
Explanation
Given:
Power of the converging lens, $P_1$ = $+$4.5 D (power of converging lens is always taken positive)
Power of the diverging lens, $P_2$ = $-$3 D (power of diverging lens is always taken negative)
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-
$P=4.5D+(-3D)$
 $P=-1.5D$
Hence, the combined power of the two lenses, $P$ is -1.5D.
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