Question

A parallel beam of light is allowed to fall on a transparent spherical globe of diameter 30 cm and refractive index 1.5. The distance from the centre of the globe at which the beam of light can converge is________ mm.

Answer 1

The correct answer is: 225

1st refraction:
$\frac{1.5}{V_1}-0=\frac{0.5}{15}$

$⇒ v_1 = 45$cm

2nd refraction:
$\frac{1}{v_2}-\frac{1.5}{15}=\frac{-0.5}{-15}$

$⇒ \frac{1}{v_2}=\frac{1}{30}+\frac{1}{10}$

$⇒ v2 = +7.5$cm

$⇒$ Distance from centre = 22.5 cm