Question

The power of a lens (biconvex) is $1.25m^{-1}$ in particular medium. Refractive index of the lens is $1.5$ and radii of curvature are $20cm$ and $40cm$ respectively. The refractive index of surrounding medium :

• A. $1.0$
• B. $\frac{9}{7}$
• C. $\frac{3}{2}$
• D. $\frac{4}{3}$

Answer 1

Answer: (B)

$P=\frac{{\mu }_2}{f}=\left({\mu }_1-{\mu }_2\right)\left(\frac{1}{R_1}-\frac{1}{R_2}\right)$
(For this formula refer to NCERT Part-2, Chapter-9, Page no. 328, solved example 8) $({\mu }_1$ is refractive index of lens and ${\mu }_2$ is of surrounding medium)
$1.25=\left(1.5-{\mu }_2\right)\left(\frac{1}{0.2}+\frac{1}{0.4}\right)$
$\frac{1.25\times 0.08}{0.6}=\left(1.5-{\mu }_2\right)$
$\Rightarrow {\mu }_2=\frac{4}{3}$