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?

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

Tardigrade Admin · Accepted Answer

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

∴

The total power of the combination

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

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

1.5 D

-1.5 D

6.5 D

-6.5 D

Power of the resultant combination is given by P=P1​+P2​ First lens is a convex lens with power P1​=+0.401​=+2.5D The second lens is a concave lens with power P2​=+0.251​=4D ∴ The total power of the combination P=P1​+P2​=2.5−4=−1.5D

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