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 $-0.5D$

For plano-concave lens: $R_1=\infty$ and $R_2=+100$ $cm$

Given: Refractive index of glass, $\mu =1.5$

Using lens maker formula:

$\Rightarrow \frac{1}{f}=(\mu -1)(\frac{1}{R_1}-\frac{1}{R_2})$

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

$\Rightarrow$ $\frac{1}{f}=-\frac{0.5}{100}$ $cm=-0.5$ $m$

$\therefore$ Power of the lens, $P=\frac{1}{f}=-0.5D$