The critical angle for a typical glass air interface is $42^\circ$. If a ray of light falls normally on one of the faces of the prism of angle $45^\circ$. The emergent ray will:

Given critical angle $C = 42^\circ$.

Angle of prism $A = 45^\circ$.

Since the ray falls normally on the first face, the angle of incidence $i_1 = 0^\circ$.

Thus, the angle of refraction at the first face $r_1 = 0^\circ$.

Using the relation for a prism, $r_1 + r_2 = A$.

Substituting the values, $0^\circ + r_2 = 45^\circ \Rightarrow r_2 = 45^\circ$.

The angle of incidence at the second face is $45^\circ$.

Since $r_2 > C$ ($45^\circ > 42^\circ$), the ray will undergo total internal reflection at the second face.

Answer: Undergo total internal reflection from the second face