Snell's Law is valid only as long as  is no greater than . When , i.e. light travels from a medium of a higher index of refraction to one with a lower refractive index, the angle  defined by

is called the critical angle. This angle is the largest possible incident angle which will create a refracted ray. As the incident angle  increases towards , the proportion of the light refracted through the surface decreases and the proportion reflected increases. When , all light is reflected, a situation known as Total Internal Reflection.