Snell's Law is valid only as long as $\sin \left({\mathrm{\&theta;}}_1\right)$ is no greater than $\frac{n_2}{n_1}$. When $n_2< n_1$, i.e. light travels from a medium of a higher index of refraction to one with a lower refractive index, the angle ${\mathrm{\&theta;}}_c$ defined by

${\mathrm{\&theta;}}_c \&equals; {\sin }^{-1}\left(\frac{n_2}{n_1}\right)$

is called the critical angle. This angle is the largest possible incident angle which will create a refracted ray. As the incident angle ${\mathrm{\&theta;}}_1$ increases towards ${\mathrm{\&theta;}}_c$, the proportion of the light refracted through the surface decreases and the proportion reflected increases. When ${\mathrm{\&theta;}}_1\ge {\mathrm{\&theta;}}_c$, all light is reflected, a situation known as Total Internal Reflection.