Why is the brilliance of a diamond observed?

$A$ diver at a depth of $12 \ m$ in water $\left(\mu = \frac{4}{3}\right)$ sees the sky in a cone of semi-vertical angle:

The phenomenon involved in the reflection of radio waves by the ionosphere is similar to:

$A$ ray of light from a denser medium strikes a rarer medium. The angle of reflection is $r$ and that of refraction is $r'$. The reflected and refracted rays make an angle of $90^{\circ}$ with each other. The critical angle will be

$A$ ray of light is incident at an angle $i$ from a denser to a rarer medium. The reflected and the refracted rays are mutually perpendicular. The angle of reflection is $r$ and the angle of refraction is $r'$. Then the critical angle $C$ will be:

Light enters at an angle of incidence $\alpha$ into a transparent rod of refractive index $n.$ For what value of the refractive index of the material of the rod will the light,once entered into it,not leave it through its lateral face,regardless of the value of the angle of incidence $\alpha$?