Biomedical Engineering Reference
In-Depth Information
What occurs when light traveling in an optically dense medium enters one
that is less dense? As can be seen in Figure 1-9B, the increase in velocity causes
the light ray to be deviated away from the normal. Again, this is a handy fact
to memorize.
It can be useful to quantify the refraction that occurs as light travels from one
medium, which we'll call the
primary medium
, into another medium, which is
called the
secondary medium
. Snell's law, which is given below, allows us to do so:
n
(
sin
θ
)
=
n
′
(
sin
θ
′
)
where
n
is the index of refraction of the primary medium,
n
′
is the index of refrac-
tion of the secondary medium,
θ
is the angle of incidence (with respect to the nor-
mal), and
θ′
is the angle of refraction (with respect to the normal).
Let's do a problem.
For a light ray traveling from air to crown glass, the angle of
incidence is 20.00 degrees. What is the angle of refraction?
In this and almost all optical problems, it's a very good idea to draw a diagram.
Figure 1-10 shows a light ray striking the glass surface such that it makes an
angle of 20 degrees with the normal to the surface. Before doing the calculation,
we know that the light ray is refracted toward the normal. How do we know this?
20.00
°
n
= 1.00
n
′
= 1.52
13.00
°
Figure 1-10.
For a light ray that strikes a crown glass surface at an angle of 20.00
degrees, the angle of refraction is 13.00 degrees.
