Question

A parallel beam of light falls on a solid transparent sphere. For what value of refractive index $\mu$, the thin beam can be focussed at circumference of sphere :

• A. 1.5
• B. 2
• C. 2.5
• D. none of these

Answer 1

Answer: (B)

Since,incident light is paralle,$u=-\infty$ and $v=2R$

$\frac{{\mu }_2}{v}-\frac{{\mu }_1}{u}=\frac{{\mu }_2-{\mu }_1}{R}$

Here, ${\mu }_2=\mu$ and ${\mu }_1=1$

$⟹\frac{\mu }{2R}-\frac{1}{-\infty }=\frac{\mu -1}{R}$

$⟹\frac{\mu }{2R}=\frac{\mu -1}{R}$

$⟹\mu =2\mu -2$

$⟹\mu =2$

Answer-(B).