Angle of incidence $\theta _{c}$ for which the refracted ray grazes the surface of contact between two media (of velocities $V_{1}$ and $V_{2}$):

$\sin {\theta _{c}}={\frac {V_{1}}{V_{2}}}$

Figure.

C-17 Critical distance. Curvature of the reflection is hyperbolic if velocity above the reflector is constant. Reflection amplitude is often large in the vicinity of critical reflection and the phase of the reflection generally changes beyond the critical distance.

Has meaning only where $V_{2}>V_{1}$.In general, four critical angles can be defined for the ratios of P- and S-waves in the two media provided they are $<1$:

${\frac {V_{P1}}{V_{P2}}},\;$
${\frac {V_{S1}}{V_{S2}}},\;$
${\frac {V_{S1}}{V_{P2}}},\;$
${\frac {V_{P1}}{V_{S2}}}\;$

The first is usually intended unless otherwise specified.