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)

-1.5 D

Answer 2

Power of the resultant combination is given b y P = P + P 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 $P_{2}=+\frac{1}{0.25}=4 D$ ? The total power of the combination P = P + P = 2.5 - 4 = - 1.5 D