Question

The power of a convex lens is $2$ dioptre. Its power is to be reduced to $1.5$ dioptre, by putting another lens in combination with it. Which of the following lenses will serve the purpose?

• A. A concave lens of focal length 2 m
• B. A concave lens of focal lens 4 m
• C. A convex lens of focal lens 2 m
• D. A concave lens of focal length 1 m

Answer 1

Answer: (A)

A concave lens of focal length 2 m

Answer 2

$P=P_{1}+P_{2}$
$P_{2}=P-P_{1}$
$=1.5-2=-0.5$
diopter $P_{2}$ is a concave lens
$f=\frac{1}{P} f_{2}=\frac{1}{0.5}=-2 \,m$