Question

In an optical instrument a convex lens of focal length 20 cm is used and kept in contact with a concave lens of focal length 40 cm, What is the power of this combination?

• A. 4.5 D
• B. 3.5 D
• C. 1.5 D
• D. 2.5 D

Answer 1

The correct option is D 2.5 D

Power of a combination of lens is equal to sum of their individual power i.e $p_{net}=p_1+p_2$
According to question
$p_1=\frac{100}{20}=5D$ [convex lens]
$p_2=\frac{100}{-40}=-2.5D$ [concave lens]
hence
$p_{net}=p_1+p_2$
$=5D-2.5D$
$p_{net}=2.5D$