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  4. A concave lens is kept in contact with a convex lens of focal length 20 cm. The combination works as convex lens of focal length 50 cm. The power of concave lens is

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Q. A concave lens is kept in contact with a convex lens of focal length $20\, cm$. The combination works as convex lens of focal length $50\, cm$. The power of concave lens is

ManipalManipal 2020

Solution:

Power of the combination
$P=P_{1}+P_{2}$
$=P_{\text {concave }}+P_{\text {convex }}$
$\Rightarrow \frac{100}{50}=P_{\text {concave }}+\frac{100}{20}$
or $2=P_{\text {concave }}+5$
or $P_{\text {concave }}=2-5=-3.0 D$