CBSE 2022 · Region 5 · Set 2 · Q12 · 5 marks

A ray of light travels from a denser to a rarer medium. After refraction, it bends away from the normal. When we keep increasing the angle of incidence, the angle of refraction also increases till the refracted ray grazes along the interface of two media. The angle of incidence for which it happens is called critical angle. If the angle of incidence is increased further the ray will not emerge and it will be reflected back in the denser medium. This phenomenon is called total internal reflection of light.

(i)

A ray of light travels from a medium into water at an angle of incidence of $\displaystyle 18^{\circ}$. The refractive index of the medium is more than that of water and the critical angle for the interface between the two media is $\displaystyle 20^{\circ}$. Which one of the following figures best represents the correct path of the ray of light ?

(i)

(C)

(D)

(ii)

A point source of light is placed at the bottom of a tank filled with water, of refractive index $\displaystyle \mu$, to a depth d. The area of the surface of water through which light from the source can emerge, is :

(a)

$\displaystyle \frac{\pi \mathrm{d}^{2}}{2\left(\mu^{2}-1\right)}$

(b)

$\displaystyle \frac{\pi \mathrm{d}^{2}}{\left(\mu^{2}-1\right)}$ c) $\displaystyle \frac{\pi \mathrm{d}^{2}}{\sqrt{2} \sqrt{\mu^{2}-1}}$

(d)

$\displaystyle \frac{2 \pi \mathrm{~d}^{2}}{\left(\mu^{2}-1\right)}$

(iii)

For which of the following media, with respect to air, the value of critical angle is maximum ?

(a)

Crown glass

(b)

Flint glass

(c)

Water

(d)

Diamond

(iv)

The critical angle for a pair of two media A and B of refractive indices $\displaystyle 2.0$ and $\displaystyle 1.0$ respectively is :

(a)

$\displaystyle 0$° (b) $\displaystyle 30$°

(c)

$\displaystyle 45$°

(d)

$\displaystyle 60$°

(v)

The critical angle of pair of a medium and air is $\displaystyle 30$°. The speed of light in the medium is :

(a)

$\displaystyle 1 \times 10^{8} \mathrm{~m} \mathrm{~s}^{-1}$

(b)

$\displaystyle 1.5 \times 10^{8} \mathrm{~m} \mathrm{~s}^{-1}$

(c)

$\displaystyle 2.2 \times 10^{8} \mathrm{~m} \mathrm{~s}^{-1}$

(d)

$\displaystyle 2.8 \times 10^{8} \mathrm{~m} \mathrm{~s}^{-1}$

CBSE Class 12 Physics past-paper question from the 2022board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.