Question

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?

• A. 1.5 D
• B. -1.5 D
• C. 6.5 D
• D. -6.5 D

Answer 1

Answer: (B)

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
$P=P_{1}+P_{2}=2.5-4=-1.5 D$