Answer
About $43^{\circ}. 24.5^{\circ}$.
Work Step by Step
The critical angle is defined to be the minimum angle of incidence at which a light ray is totally reflected, back into the medium.
For glass in air, the critical angle is about $43^{\circ}$, depending on the particular type/composition of glass.
For a diamond in air, the critical angle is $24.5^{\circ}$.
This is discussed on pages 532-533.