Question

A plano-concave lens is made of glass of refractive index $1.5$ and the radius of curvature of its curved face is $100 \,cm$. What is the power of the lens?

• A. +0.5 D
• B. -0.5 D
• C. -2D
• D. +2D

Answer 1

Answer: (B)

-0.5 D

Answer 2

The refractive index is $1.5$ and the radius of curvature of the curved face is $100\, cm$ and of the plane side is $\infty ,$ hence
$P=\frac{1}{f}=(\mu-1)\bigg(\frac{1}{R_1}-\frac{1}{R_2}\bigg)$
$=(1.5-1)\bigg(\frac{1}{\infty}-\frac{1}{1}\bigg)$
$= -0.5\, D$