illuminated from below. Ray PBB+ strikes the clear surface at
the critical angle and is totally reflected, as are rays such as
PCC+. Rays such as PAA+ emerge from the clear surface. On
the painted surface there appears a dark circle of diameter
d, surrounded by an illuminated region, or halo. (a) Derive
an equation for n in terms of the measured quantities d and
t. (b) What is the diameter of the dark circle if n ! 1.52 for a
slab 0.600 cm thick? (c) If white light is used, the critical
angle depends on color caused by dispersion. Is the inner
edge of the white halo tinged with red light or violet light?
Explain.
70.
A light ray traveling in air is incident on one face of a
right-angle prism of index of refraction n ! 1.50 as shown
in Figure P35.70, and the ray follows the path shown in the
figure. Assuming & ! 60.0° and the base of the prism is
mirrored, determine the angle 0 made by the outgoing
ray with the normal to the right face of the prism.
Answers to Quick Quizzes
1125
Figure P35.70
Figure P35.71
Incoming ray
θ
Outgoing ray
Mirror base
n
φ
90
° –
θ
n
2
L
1
θ
θ
incidence versus the sine of the angle of refraction. Use the
resulting plot to deduce the index of refraction of water.
Angle of Incidence
Angle of Refraction
(degrees)
(degrees)
10.0
7.5
20.0
15.1
30.0
22.3
40.0
28.7
50.0
35.2
60.0
40.3
70.0
45.3
80.0
47.7
Answers to Quick Quizzes
35.1 (d). The light rays from the actor’s face must reflect from
the mirror and into the camera. If these light rays are
reversed, light from the camera reflects from the mirror
into the eyes of the actor.
35.2 Beams " and $ are reflected; beams # and % are
refracted.
35.3 (c). Because the light is entering a material in which the
index of refraction is lower, the speed of light is higher
and the light bends away from the normal.
35.4 (a). Due to the refraction of light by air, light rays
from the Sun deviate slightly downward toward the
surface of the Earth as the light enters the atmosphere.
Thus, in the morning, light rays from the upper edge of
the Sun arrive at your eyes before the geometric line
from your eyes to the top of the Sun clears the horizon.
In the evening, light rays from the top of the Sun con-
tinue to arrive at your eyes even after the geometric line
from your eyes to the top of the Sun dips below the
horizon.
35.5 (c). An ideal camera lens would have an index of refrac-
tion that does not vary with wavelength so that all colors
would be bent through the same angle by the lens. Of the
three choices, fused quartz has the least variation in n
across the visible spectrum.
35.6 (b). The two bright rays exiting the bottom of the prism
on the right in Figure 35.27 result from total internal
reflection at the right face of the prism. Notice that there
is no refracted light exiting the slanted side for these rays.
The light from the other three rays is divided into
reflected and refracted parts.
35.7 (b). Counterclockwise rotation of the prism will cause the
rays to strike the slanted side of the prism at a larger angle.
When all five rays strike at an angle larger than the critical
angle, they will all undergo total internal reflection.
35.8 (c). When the outgoing beam approaches the direction
parallel to the straight side, the incident angle is
approaching the critical angle for total internal reflection.
The index of refraction for light at the violet end of the
visible spectrum is larger than that at the red end. Thus,
as the outgoing beam approaches the straight side, the
violet light experiences total internal reflection first,
followed by the other colors. The red light is the last to
experience total internal reflection.
71.
A light ray enters a rectangular block of plastic at an angle
&
1
!
45.0° and emerges at an angle &
2
!
76.0°, as shown
in Figure P35.71. (a) Determine the index of refraction of
the plastic. (b) If the light ray enters the plastic at a point
L ! 50.0 cm from the bottom edge, how long does it take
the light ray to travel through the plastic?
72.
Students allow a narrow beam of laser light to strike a
water surface. They arrange to measure the angle of
refraction for selected angles of incidence and record the
data shown in the accompanying table. Use the data to verify
Snell’s law of refraction by plotting the sine of the angle of