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:

30°

The correct answer is Option (3) → 30°

The refractive index of a medium is -

$μ=\frac{velocity\,in\,rarer\,medium}{velocity\,in\,denser\,medium}$

$∴μ=\frac{2v}{v}$ [v → velocity in denser medium]

$μ=2$

and,

Critical angle, $θ_c⇒\sin θ_c=\frac{1}{μ}$

$⇒\sin θ_c=\frac{1}{2}$

$∴θ_c=30°$