The phenomenon of refraction results from light traveling at different speeds in different
media. The “speed of light” constant c is really the speed of light in a vacuum: when light
passes through matter, it slows down. If light travels through a substance with velocity v,
then that substance has an index of refraction of n = c/v. Because light always travels
slower through matter than through a vacuum, v is always less than or equal to c, so
. For transparent materials, typical values of n are quite low: = 1.0, = 1.3, and
= 1.6. Because it is the presence of matter that slows down light, denser materialsgenerally have higher indices of refraction.
A light ray passing from a less dense medium into a denser medium will be refracted
toward the normal, and a light ray passing from a denser medium into a less dense
medium will be refracted away from the normal. For example, water is denser than air, so
the light traveling out of water toward our eyes is refracted away from the normal. When
we look at a straw in a glass of water, we see the straw where it would be if the light had
traveled in a straight line.