Question

A thin glass (refractive index 1.5) lens has optical power of -8 D in air. Its optical power in a liquid medium with refractive index 1.6 will be :

• A. 1 D
• B. -1 D
• C. 25 D
• D. -25 D

Answer 1

The correct option is A 1 D

In air,

$P=\frac{1}{f}=[\frac{{\mu }_g}{{\mu }_a}-1][\frac{1}{R_1}-\frac{1}{R_2}]$

In medium

$P^{\prime }=\frac{1}{f^{\prime }}=[\frac{{\mu }_g}{{\mu }_l}-1][\frac{1}{R_1}-\frac{1}{R_2}]$

$\frac{P^{\prime }}{P}=\frac{[\frac{{\mu }_g}{{\mu }_l}-1]}{[\frac{{\mu }_g}{{\mu }_l}-1]}=\frac{[\frac{1.5}{1.6}-1]}{[\frac{1.5}{1}-1]}=\frac{(-0.1/1.6)}{0.5}$

$P^{\prime }=1D$