Indices of refraction
Green light travels at 1.99×10^8
m/s in crown glass. What is the
index of refraction of the glass
for this light?
n = 1.51
32.3 - Snell’s law
Snell’s law is used to quantify refraction. In doing so, it uses some of the same
terminology as the law of reflection. As with reflection, angles are measured between a
ray and a line normal (perpendicular) to a surface. You see this illustrated in Concept 1,
with both the angle of incidence (și) and the angle of refraction (șr) shown.
Snell’s law, shown in Equation 1, expresses the relationship between these angles. This
law was discovered empirically by Willebrord Snell and written in its current form by
René Descartes. It states that the product of the sine of the incident angle and the index
of refraction of the incident medium equals the product of the sine of the refraction
angle and the index of refraction of the refracting medium.
We also show the same equation in an alternate formulation: The ratio of the sine of the
incident angle to the sine of the refracted angle is the reciprocal of the ratio of the first
to the second index of refraction.
To put it more concretely, light bends toward the normal when it slows down, for
instance, when it passes from air to water. You see this in Concept 1 to the right. It
bends away from the normal when it speeds up, as from water to air.
If light passes through several media, Snell’s law can be applied at each interface. You see this occurring in Concept 2. The light’s direction
changes toward the normal at the first interface as it slows, and then away from the normal as it crosses the second interface. It bends away
because light moves faster in the third medium, which has a lesser index of refraction than the second.
Snell’s law
Quantifies refraction
Slower light bends toward normal
(^598) Copyright 2007 Kinetic Books Co. Chapter 32