Question

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

• A. 1
• B. $\frac{9}{7}$
• C. $\frac{3}{2}$
• D. $\frac{4}{3}$

Answer 1

Answer: (B)

$\frac{9}{7}$

Answer 2

$P=\frac{μ_2}{f}=(μ_1-μ_2)(\frac{1}{R_1}-\frac{1}{ R_2})$

here, ($μ_1$is refractive index of lens and $μ_2$ is of surrounding medium)

$1.25μ_2= (1.5-μ_2)(\frac{1}{0.2}+\frac{1}{0.4})$

⇒ $μ_2=\frac{9}{7}$