Question

Convex lens of power 4D and a concave lens of power 3D are placed in contact. What is the equivalent power of the combination?

• A. $\frac{3}{4}D$
• B. $7D$
• C. $\frac{4}{3}D$
• D. $1D$

Answer 1

Answer: (D)

$1D$

Answer 2

Here: Power of convex lens = 4D Power of concave lens = 3D
Hence, die equivalent power of the combination = 4D - 3D = 1D
( $\because$ Concave lens has negative (minus) focal length, so its power will also be negative.)