Cubic zirconia also has a high index of refraction and can be made to sparkle very
much like a genuine diamond. If a suspect jewel is immersed in corn syrup, the differ-
ence in n for the cubic zirconia and that for the syrup is small, and the critical angle is
therefore great. This means that more rays escape sooner, and as a result the sparkle
completely disappears. A real diamond does not lose all of its sparkle when placed in
corn syrup.
S E C T I O N 3 5 . 8 • Total Internal Reflection
1113
Figure 35.28 (Example 35.8) What If? A fish looks upward
toward the water surface.
Quick Quiz 35.6
In Figure 35.27, five light rays enter a glass prism from the
left. How many of these rays undergo total internal reflection at the slanted surface of
the prism? (a) 1 (b) 2 (c) 3 (d) 4 (e) 5.
Quick Quiz 35.7
Suppose that the prism in Figure 35.27 can be rotated
in the plane of the paper. In order for all five rays to experience total internal
reflection from the slanted surface, should the prism be rotated (a) clockwise or
(b) counterclockwise?
Quick Quiz 35.8
A beam of white light is incident on a crown glass–air
interface as shown in Figure 35.26a. The incoming beam is rotated clockwise, so that
the incident angle & increases. Because of dispersion in the glass, some colors
of light experience total internal reflection (ray 4 in Figure 35.26a) before other
colors, so that the beam refracting out of the glass is no longer white. The last color
to refract out of the upper surface is (a) violet (b) green (c) red (d) impossible
to determine.
Figure 35.27 (Quick Quiz 35.6
and 35.7) Five nonparallel light
rays enter a glass prism from the
left.
Courtesy of Henry Leap and Jim Lehman
Find the critical angle for an air–water boundary. (The
index of refraction of water is 1.33.)
Solution We can use Figure 35.26 to solve this problem,
with the air above the water having index of refraction
n
2
and the water having index of refraction n
1
. Applying
Equation 35.10, we find that
What If?
What if a fish in a still pond looks upward toward
the water’s surface at different angles relative to the surface,
as in Figure 35.28? What does it see?
Answer Because the path of a light ray is reversible, light
traveling from medium 2 into medium 1 in Figure 35.26a
follows the paths shown, but in the opposite direction. A fish
looking upward toward the water surface, as in Figure 35.28,
can see out of the water if it looks toward the surface at an
angle less than the critical angle. Thus, for example, when
the fish’s line of vision makes an angle of 40° with the
normal to the surface, light from above the water reaches
the fish’s eye. At 48.8°, the critical angle for water, the light
has to skim along the water’s surface before being refracted
to the fish’s eye; at this angle, the fish can in principle see
the whole shore of the pond. At angles greater than the
critical angle, the light reaching the fish comes by means of
internal reflection at the surface. Thus, at 60°, the fish sees a
reflection of the bottom of the pond.
48.8/
&
c
!
sin
&
c
!
n
2
n
1
!
1
1.33
!
0.752
θ
Example 35.8 A View from the Fish’s Eye