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?

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

Tardigrade Admin · Accepted Answer

P=P1+P2 P2=P-P1 =1.5-2=-0.5 diopter P2 is a concave lens f=(1/P) f2=(1/0.5)=-2 m

Q. 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 concave lens of focal length 2 m

A concave lens of focal lens 4 m

A convex lens of focal lens 2 m

A concave lens of focal length 1 m

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

Q. 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?