Law of Refraction
Just like the car would in the previous example,
light moves slower as it passes through denser media.* What
Snellius realized is that there is a correlation between the
speed of light through a substance and the angle of
refraction. He eventually formulated this into Snell�s Law,
shown below.
For this equation to work,
theta must be measured from the normal angle (the one
perpendicular to the surface). In this case, n in the
ratio of the speed of light in a vacuum and speed of light
in that the specific material, known as the Refractive
Index. Because light can�t move any faster than it does in
a vacuum, this number will always be more than one. Most
materials have a Refractive Index of somewhere between 1
and 2.5.
From: Wikimedia Foundation
*Density isn�t the only
contributing factor, but it is generally the best
indicator. Other qualities that affect the speed of
light thru a medium include structure and composition of
material.