Given a ray traveling from a medium with index of refraction into a medium with index
of refraction , Snell’s Law governs the relationship between the angle of incidence
and the angle of refraction:
EXAMPLE
A ray of light passes from a liquid medium into a gas medium. The incident ray has an angle
of 30º with the normal, and the refracted ray has an angle of 60º with the normal. If light
travels through the gas at a speed of m/s, what is the speed of light through the
liquid medium? sin 30º = 0.500 and sin 60º = 0.866.We know that the index of refraction for a substance, n, gives the ratio of the speed of
light in a vacuum to the speed of light in that substance. Therefore, the index of
refraction, , in the liquid medium is related to the speed of light, , in that medium by
the equation = c/ ; similarly, the index of refraction, , in the gas medium is related
to the speed of light, , in that medium by the equation = c/. The ratio between
and is:
We can calculate the ratio between and using Snell’s Law: