Ch. 22
Concept questions # 2, 7, 14, 15
Problems #1, 6, 7, 11, 19, 12, 13, 21,
29, 33, 38, 40, 47
Nature of Light
Up until the 19th century, light was modeled as a
stream of particles.
Provided simple explanations for reflection and
refraction.
Another theory was that light behaved like a wave.
Not as widely accepted.
In 1801, Thomas Young showed that light traveling
along two different paths produced interference.
This could not be explained by the particle
theory.
It turns out that light has some behaviors
associated with particles, while other behaviors
are wave properties.
Light travels in a straight-line path in a
homogeneous medium, until it encounters a
boundary between two different materials.
When the light hits the boundary, it either is
reflected from the boundary, passes into the
medium on other side of boundary, or partially
does both.
Model light as a ray traveling in the direction of the
light beam. When light rays travel in parallel
paths, a wave front in the shape of a plane is
produced.
When light encounters a boundary, part of the
incident beam is reflected back into the first
medium.
Reflection from a smooth surface is called specular
reflection. The reflected rays are parallel to each
other.
Reflection from a rough surface is diffuse reflection.
The rays reflect in many directions.
A surface is smooth if variations of the surface are
small compared to the wavelength of the incident
light.
see fig. 22.2
Reflection
If a line is drawn perpendicular to the boundary,
we call this line, the normal.
Measure angles of incidence and reflection as
the angle between the light and the normal
line.
Law of reflection: The angle of reflection
equals the angle of incidence.
or = ’
i= r
See fig. 22.4
Refraction
What happens to the light that crosses the
boundary between mediums.
When light goes from one medium to another,
the light can be bent. It will be bent as long as
it does not hit the boundary perpendicular to
the surface.
see pictures on page 731
Refraction
Index of refraction: defined as the ratio of the
speed of light in vacuum to the speed of light in
the medium.
n = c/v
c = speed of light in vacuum 3 x 108 m/s
Because v is equal or less than c, n is equal to or
greater than one.
n = 1 for vacuum
table on page 732
As light travels from one medium to another, the
frequency does not change.
So for the speed to change, the wavelength has to
change.
See figure 22.8
Frequency is constant:
v1 = f 1
and
v2f 2
solving for f and setting equal to each other we get:
v1/ 1 = v2/ 2
1
2
n
1 1
v1
v2
c / n1
c / n2
n
2 2
n2
n1
• Index of refraction, n is the ratio of the
wavelength in vacuum to the wavelength in the
medium.
n = 0/ n
Snell’s law of refraction. Used to relate angle of
incidence to angle of refraction.
n1 sin 1 = n2 sin 2
If light goes from higher n to lower n, the light
bends away from the normal.
If light goes from lower n to higher n, the light
bends closer to the normal.
• See quick quiz 22.3
• see example of light passing through a slab.
(22.4)
• http://www.ps.missouri.edu/rickspage/refract
/refraction.html
Dispersion and prisms
• Dispersion – the dependence of index of
refraction on the wavelength.
• The colors of the visible spectrum each have
their own wavelength. So different colors
(wavelengths) refract more than others.
see figure 22.14
White light is made up of all the colors in the
visible spectrum.
When white light enters a prism, the different
colors get refracted (bent) by different angles.
Thus the prism splits up the white light into
the different colors.
See pictures on page 737.
Rainbows are a result of dispersion. Drops of
water take the place of the prism.
See fig. 22.19.
Huygens’ Principle
Geometric method of describing reflection
and refraction. Uses wave nature of light.
Used to determine a new wave front from the
previous wave front.
All points on a given wave front are the sources
of spherical waves (called wavelets). After
some time has elapsed, the new wave front is
tangent to the wavelets.
fig. 22.21
Huygens’ principle applied to ocean waves.
Before hitting the breakwalls, the waves are
plane waves. After going through, the gaps
they are circular waves. The waves at the gaps
behave as sources for the circular waves.
fig. 22.22
Total internal reflection
Can occur when light goes from medium with
higher index of refraction to medium with
lower index of refraction.
When the angle of incidence is past a critical
value, all the light will be reflected.
The rays will obey the law of reflection.
To find the critical angle set the angle of
refraction to 900.
n1 sin
sin
c
c
= n2 sin 90
= n2/n1
for n1 > n2
see picture on page 742, 743
do example 22.6
Fiber optics
Uses total internal reflection to carry light
from one location to another with losing very
little intensity.
see figures 22.29 and 22.30
Applications: looking inside people, carrying
information