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.