Question

Diagram alongside shows a glass slab, such that a ray of light AB is incident on it at an angle of near 90°. If the critical angle for glass is 42°, trace the course of ray AB, through glass, till it emerges out in air. Copy the diagram.

Answer 1

Ray AB strikes the face at B at nearly 90° (grazing incidence), so it refracts into the glass at an angle of 42°, equal to the critical angle.

The two faces of the slab are parallel, so the refracted ray BC meets the lower face at C at 42°, which again equals the critical angle.

Since the angle of incidence at C equals the critical angle, the angle of refraction there is 90°. The ray CD therefore leaves the slab grazing along its lower surface.