Give a condition when Snell's law fails.

Snell's law gives the relation between the angle of
incidence and the angle of refraction when a beam of light moves from one medium to
another. If the angle of incidence is Ai and the angle of refraction is Ar, the two are
related by sin Ai / sin Ar = V1 / V2, where V1 and V2 is the velocity of light in the
first and second medium respectively.

The law does not give
correct values in case there is total internal reflection. Total internal reflection
occurs when the ratio of the speed of light in the second medium to that in the first
medium is greater than 1. For example if the ratio of the velocities V2/ V1 is given by
1.5 and the angle of incidence is 60 degree, we have sin Af = 1.5 * 0.8660 = 1.299 which
is not possible as the value of the sine of an angle cannot exceed
1.