Question

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

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

Answer 1

The correct option is B $-0.5D$

Radius of curvature of plane surface is infinity i.e. $R_1=\infty$

Radius of curvature of plane surface $R_2=100cm$

Refractive index of the glass $\mu =1.5$

Form lens maker formula, $\frac{1}{f}=(\mu -1)(\frac{1}{R_1}-\frac{1}{R_2})$

$\therefore$ $\frac{1}{f}=(1.5-1)(\frac{1}{\infty }-\frac{1}{100})$

$⟹$ $\frac{1}{f}=\frac{-0.5}{100}c{m}^{-1}=-0.5m^{-1}$

Power of the lens $P=\frac{1}{f}$ where $f$ is in metres.

$⟹$ $P=-0.5D$