Question

Surfaces of a thin equi-convex glass lens $(\mu =1.5)$ have radius of curvature $R$. Paraxial rays are incident on it. If the final image is formed at a distance $\frac{R}{13}$ from pole of the lens after $n$ internal reflection. Then n is:

• A. 2
• B. 3
• C. 4
• D. $Infinity$

Answer 1

The correct option is A 2

Surface of a thin equi-convex

glass lens$=\mu =1.5$

Radius of curvature $=R$

Paraxial rays are incident on it

Final image is formed at a distance

$=\frac{R}{13}$ from pole of the lens after n internal reflection.

Solution

$\frac{1}{f}=(\mu -1)(\frac{1}{R_1}-\frac{1}{R_2})\frac{1}{f}=(1.5-1)(\frac{1}{R}+\frac{1}{R})=0.5\frac{2}{R}=\frac{5}{10}\times \frac{2}{R}=\frac{1}{R}f=R\frac{1}{f}=\frac{13}{nR}=\frac{2}{R}n=2$