## Conceptual Physics (12th Edition)

About $43^{\circ}. 24.5^{\circ}$.
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.