Critical Angle
The specific angle of incidence in an optically denser medium for which the angle of refraction in the rarer medium is exactly 90 degrees.
Governing formula
θ_c = arcsin(n2 / n1)
SI unit
Degrees (°)
In depth
At $θ_i = θ_c$, the refracted ray grazes the interface ($90^°$). For any $θ_i > θ_c$, total internal reflection occurs exclusively.
Examples in the real world
The critical angle for a crown glass to air interface is approximately 41.8°, while for diamond to air it is 24.4°.