Physics 1161: Lecture 22
Refraction
• Textbook sections 26-3 – 26-5, 26-8
Wave Boundary Behavior
• wave speed and wavelength are greater in less
dense medium
• wave frequency is not altered by crossing boundary
• reflected pulse is inverted when wave travels from
less dense medium to more dense medium
• incident pulse amplitude is greater than reflected
pulse amplitude
Transmission Across a Boundary
• Wave speed & wavelength change
• When the wave approach is perpendicular to
the boundary, its speed changes, but there is
no bending of the path
Refraction of Light Beam
• Refraction -- bending of light wave path as light
passes from one material to another material.
• Refraction occurs at the boundary and is caused by a
change in the speed of the light wave upon crossing
the boundary.
• Direction of bending depends upon whether light
wave speeds up or slows down at the boundary.
Optical Density
• Optical density -- tendency of the atoms
of a material to hold on to absorbed
energy from a photon in the form of
vibrating electrons before reemitting it as
a new photon
• The more optically dense a material is, the
slower a wave will move through the
material.
Index of Refraction
• Index of Refraction is a measure of optical
density
• Represented by
n
• The higher n is, the more optically dense the
material and the slower light travels in the
material
Indices of Refraction
Physics 1161: Lecture 17, Slide 7
Law of Refraction
Snell’s Law
n1sinq1 = n2sinq2
Refraction Applets
• Applet by Molecular Expressions -- Florida
State University
• Applet by Fu-Kwung Hwang, National Taiwan
Normal University
FST & SFA
• A ray of light crossing the boundary from
a fast medium to a slow medium bends
toward the normal. (FST)
• A ray of light crossing the boundary from
a slow medium to a fast medium bends
away from the normal. (SFA)
Snell’s Law
When light travels from one medium to another the
speed changes v=c/n, but the frequency is constant.
So the light bends:
n1 sin(q1)= n2 sin(q2)
n1
q1
q2
Compare n1 to n2.
n2
Preflight 22.1
1) n1 > n2
21 %
2) n1 = n2
21 %
3) n1 < n2
58 %
Snell’s Law
When light travels from one medium the speed
changes. If the angle of incidence is greater than 0,
the light bends. During this process, the frequency
remains constant.
n1 sin(q1)= n2 sin(q2)
n1
q1
q2
Compare n1 to n2.
n2
Preflight 22.1
1) n1 > n2
q1 < q2
2) n1 = n2
sinq1 < sinq2
3) n1 < n2
n1 > n2
Snell’s Law Practice
Usually, there is both reflection and refraction!
A ray of light traveling through the air (n=1) is incident on water
(n=1.33). Part of the beam is reflected at an angle qr = 60. The
other part of the beam is refracted. What is q2?
q1
qr
n1
n2
normal
n1 sin q1  n2 sin q2
q
2
Snell’s Law Practice
Usually, there is both reflection and refraction!
A ray of light traveling through the air (n=1) is incident on water
(n=1.33). Part of the beam is reflected at an angle qr = 60. The
other part of the beam is refracted. What is q2?
q1 =qr =60
q1
qr
sin(60) = 1.33 sin(q2)
q2 = 40.6 degrees
n1
n2
normal
n1 sin q1  n2 sin q2
q
2
Parallel light rays cross interfaces from air
into two different media, 1 and 2, as shown
in the figures below. In which of the media
is the light traveling faster?
1. Medium 1
2. Medium 2
air
air
3. Both the same
1
2
0%
1
0%
2
0%
3
Parallel light rays cross interfaces from air
into two different media, 1 and 2, as shown
in the figures below. In which of the media
is the light traveling faster?
1. Medium 1
2. Medium 2
air
air
3. Both the same
1
2
The greater the difference in the speed
of light between the two media, the
greater the bending of the light rays.
0%
1
0%
2
0%
3
Parallel light rays cross interfaces from medium 1
into medium 2 and then into medium 3. What can
we say about the relative sizes of the indices of
refraction of these media?
1. n1 > n2 > n3
2. n3 > n2 > n1
3. n2 > n3 > n1
4. n1 > n3 > n2
5. none of the
above
1
2
3
0%
1
0%
0%
2
3
0%
0%
4
5
Parallel light rays cross interfaces from medium 1
into medium 2 and then into medium 3. What can
we say about the relative sizes of the indices of
refraction of these media?
1
1. n1 > n2 > n3
2. n3 > n2 > n1
3. n2 > n3 > n1
4. n1 > n3 > n2
2
3
5. none of the
above
Rays are bent toward the normal when crossing into #2, so n
0%
1
0%
0%
2
3
0%
0%
4
5
> n 1.
But rays are bent away from the normal when going into #3, so n3 < n2.
2
How to find the relationship between #1 and #3? Ignore medium #2!
So the rays are bent away from the normal if they would pass from
#1 directly into #3. Thus, we have: n2 > n1 > n3 .
Apparent Depth
• Light exits into
medium (air)
of lower index
of refraction,
and turns left.
Spear-Fishing
• Spear-fishing is made
more difficult by the
bending of light.
• To spear the fish in the
figure, one must aim at
a spot in front of the
apparent location of the
fish.
To spear a fish, should you aim directly at
the image, slightly above, or slightly
below?
1. aim directly at the image
2. aim slightly above
3. aim slightly below
0%
1
0%
2
0%
3
To spear a fish, should you aim directly at
the image, slightly above, or slightly
below?
1. aim directly at the image
2. aim slightly above
3. aim slightly below
0%
1
Due to refraction, the image will
appear higher than the actual
fish, so you have to aim lower to
compensate.
0%
2
0%
3
To shoot a fish with a laser gun, should
you aim directly at the image, slightly
above, or slightly below?
1. aim directly at the image
2. aim slightly above
3. aim slightly below
0%
1
The light from the laser beam
will also bend when it hits the
air-water interface, so aim
directly at the fish.
0%
2
0%
3
Delayed Sunset
• The sun actually
falls below below
the horizon
• It "sets", a few
seconds before
we see it set.
Broken Pencil
Water on the Road Mirage
Palm Tree Mirage
Mirage Near Dana – Home of Ernie Pyle