When a ray of light enters from denser medium to rarer medium, its velocity is doubled. The critical angle for total internal reflection will be:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 30°
The critical angle \( C \) is the angle of incidence in the denser medium for which the angle of refraction in the rarer medium is \( 90^\circ \). It is fixed by the refractive index of the denser medium with respect to the rarer medium.
\( \displaystyle \sin C = \frac{1}{n_{dr}}, \qquad n_{dr} = \frac{v_{\mathrm{rarer}}}{v_{\mathrm{denser}}} \)
The speed doubles on entering the rarer medium, so \( v_{\mathrm{rarer}} = 2\,v_{\mathrm{denser}} \).
\( \displaystyle n_{dr} = \frac{2\,v_{\mathrm{denser}}}{v_{\mathrm{denser}}} = 2 \)
\( \displaystyle \sin C = \frac{1}{2} \;\Rightarrow\; C = 30^\circ \)
Note: The same question was also asked in CUET (UG) 2026 on 18-05-2026 (Slot 1).