The refractive index of diamond is $2.42 .$ What is the critical angle for light passing from diamond to air?
We use $n_{i} \sin \theta_{i}=n_{t} \sin \theta_{t}$ to obtain
$$
(2.42) \sin \theta_{c}=(1) \sin 90^{\circ}
$$
from which it follows that $\sin \theta_{c}=0.413$ and $\theta_{c}=24.4^{\circ}$.