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  4. A doctor advices a patient to use spectacles with a convex lens of focal length 40 cms in contact with a concave lens of focal length 25 cms. What is the power of the resultant combination?

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Q. A doctor advices a patient to use spectacles with a convex lens of focal length $40\, cms$ in contact with a concave lens of focal length $25\, cms$. What is the power of the resultant combination?

AIIMSAIIMS 2000

Solution:

Power of the resultant combination is given by
$P=P_{1}+P_{2}$
First lens is a convex lens with power
$P_{1}=+\frac{1}{0.40}=+2.5 D$
The second lens is a concave lens with power
$P_{2}=+\frac{1}{0.25}=4 D$
$\therefore $ The total power of the combination
$P=P_{1}+P_{2}=2.5-4=-1.5 D$