Laser Tracking on the CD Scaled Views of a Compact Disc

For light from a medium of index =
incident upon a medium of index =
at an angle = °
the angle of transmission is = °
Fresnel's equations give the reflection coefficients:
= and =
The transmission coefficients are
= and
=
Note that these coefficients are fractional amplitudes, and must be squared to get fractional
intensities for reflection and transmission.
You can choose values of parameters which will give transmission coefficients greater than 1, and
that would appear to violate conservation of energy. (For example, try light incident from a medium
of n 1 =1.5 upon a medium of n 2 =1.0 with an angle of incidence of 30°.) But the square of the
transmission coefficient gives the transmitted energy flux per unit area (intensity), and the area of
the transmitted beam is smaller in the refracted beam than in the incident beam if the index of
refraction is less than that of the incident medium. When you take the intensity times the area for
both the reflected and refracted beams, the total energy flux must equal that in the incident beam.
For further details, see Jenkins and White.
Checking out conservation of energy in this situation leads to the relationship