Textbook sections 26-3

1
Refraction
Physics 1161 Lecture 22

• Textbook sections 26-3 26-5, 26-8

2
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

3
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

4
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.

5
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.

6
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

7
Indices of Refraction
8
Law of RefractionSnells Law

• n1sinq1 n2sinq2

9
Refraction Applets

• Applet by Molecular Expressions -- Florida State
  University
• Applet by Fu-Kwung Hwang, National Taiwan Normal
  University

10

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)

11
Snells Law
When light travels from one medium to another the
speed changes vc/n, but the frequency is
constant. So the light bends
n1 sin(q1) n2 sin(q2)
Preflight 22.1
n1
q1
1) n1 gt n2 2) n1 n2 3) n1 lt n2
21
q2
n2
21
Compare n1 to n2.
58
12
Snells 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)
Preflight 22.1
n1
q1
1) n1 gt n2 2) n1 n2 3) n1 lt n2
q1 lt q2
q2
n2
sinq1 lt sinq2
Compare n1 to n2.
n1 gt n2
13
Snells Law Practice
Example
Usually, there is both reflection and refraction!
A ray of light traveling through the air (n1) is
incident on water (n1.33). Part of the beam is
reflected at an angle qr 60. The other part of
the beam is refracted. What is q2?
n1
n2
normal
14
Snells Law Practice
Example
Usually, there is both reflection and refraction!
A ray of light traveling through the air (n1) is
incident on water (n1.33). 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
sin(60) 1.33 sin(q2)
n1
q2 40.6 degrees
n2
normal
15
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
3. Both the same

2
16
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
3. Both the same

2
17
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 gt n2 gt n3
• 2. n3 gt n2 gt n1
• 3. n2 gt n3 gt n1
• 4. n1 gt n3 gt n2
• 5. none of the above

18
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 gt n2 gt n3
• 2. n3 gt n2 gt n1
• 3. n2 gt n3 gt n1
• 4. n1 gt n3 gt n2
• 5. none of the above

Rays are bent toward the normal when crossing
into 2, so n2 gt n1. But rays are bent away
from the normal when going into 3, so n3 lt n2.
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 gt n1 gt n3 .
19
Apparent Depth

• Light exits into medium (air) of lower
  index of refraction,  and turns left.

20
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.

21
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

22
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

23
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

light from fish
laser beam
The light from the laser beam will also bend
when it hits the air-water interface, so aim
directly at the fish.
24
Delayed Sunset

• The sun actually falls below below the horizon
• It "sets", a few seconds before we see it set.

25
Broken Pencil
26
Water on the Road Mirage
27
Palm Tree Mirage
28
Mirage Near Dana Home of Ernie Pyle
29
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