Physics - Geometrical Optics Question with Solution | TestHub

Ordinary refraction cannot bend a ray by $90^{\circ }$ back into the same medium; the required deviation must come from total internal reflection at the second face with $i=45^{\circ }$.

TIR at $45^{\circ }$ is possible only if the critical angle for the prism-air pair satisfies $C\le 45^{\circ }$.