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Physics 2112
Unit 25
Topic 25, Slide 1
What happens at boundaries
- Reflection
- Refraction
- Snell’s Law
- Critical Angle
Polarization
- Brewster’s Angle
Recall from 2111: When wave hits a boundary where speed changes - part is transmitted - part is reflected DEMO
Boundaries
Topoc 25, Slide 2
Same with E&M: Think of window. Can see through it Can see reflection in it
Speed of light is different in
vacuum and glass
Relative speeds given by index of refraction, n
Speed of Light in media
Topic 25, Slide 3
n = speed of light in vacuum
speed of light in media
Media n
vacuum 1.0
air ~1.0
water 1.33
alcohol 1.36
glass 1.5
diamond 2.4
= V
C
oo
c
1
Recall:
/oo
So can think of n as:
Reflection
Topic 25, Slide 4
Measured from
normal
CheckPoint 2: Changes at Boundary
Topic 25, Slide 5
Which of the following quantities does not change as the light
enters the water?
A. wavelength
B. frequency
C. speed of propagation
D. (A) and (B)
E. (B) and (C)
A ray of light passes from air into water with an angle of incidence of
30 degrees.
30o
Refraction: Snell’s Law
Unit 25, Slide 6
1122 sinsin nn 1
1
2
2
/
sin
/
sin
nc
D
nc
D
D
a
b c
d
tab = tdc
tab=Dxab/v1
Not the quickest route…
This one is better
What's the fastest path to the ball knowing you can run faster than you can swim?
Think of it another (weird) way…
Topic 25, Slide 7
Minimum Travel Time
A
B
x1
y1
l1
x2
y2 l2
Setting dt/dx1 0
Same Principle works for Light! D
Topic 25, Slide 8
v c/n 2211 sinsin nn 2
2
1
1 sinsin
vv
0
22
2
11
1 lv
x
lv
x
How is x2 related to x1? 12 xDx 1
1
2 dx
dx
To find minimum time, differentiate t wrt x1 and set 0
1
2
2
2
2
22
2
2
1
2
11
1
1 dx
dx
yxv
x
yxv
x
dx
dt
Time from A to B : 2
2
2
2
2
1
2
1
2
1
2
2
1
1
v
yx
v
yx
v
l
v
lt
Not on test….
REALLY(!) weird (….but not on test)
This business of “minimum travel
time” is everywhere in nature!
Remember from 2111? When you kick a soccer
ball up at 30o, where does it land?
It lands so that the integral has
minimum value. dsPEKE )(
“Principle of Least Action” “Lagranginan”
A ray of light is bent as it passes from medium 1 to medium 2.
Compare the indices of refraction in
the two mediums.
A. n1 > n2
B. n1 = n2
C. n1 < n2
Topic 25, Slide 10
CheckPoint 2: Refraction
How much is the beam shifted by the
cube?
Example 25.1: Cube of Glass
Topic 25, Slide 11
25o
?
A beam of laser light hits a cube of glass
that is 4cm on a side with a angle that is
25o to the normal. The index of
refraction for glass is n= 1.5.
At what angle will it leave the cube?
?
The light from a fish is
incident from below on the
boundary between water and
air with an incident angle of
30o.
At what refracted angle does
it leave the boundary?
Example 25.2: Fish in the water
Topic 25, Slide 12
What if the incident angle had been 60o?
30o
Topic 25, Slide 13
Phet Demo
Click Here for Demo
1 > c Total Internal Reflection
NOTE: n1 > n2 implies 2 > 1 BUT: 2 has max value = 90o!
Total Internal Reflection
Unit 25, Slide 14
Example 25.3: Diver’s Ilusion
Unit 25, Slide 15
If a scuba diver looks straight up towards the
surface from underwater, he see a bright circle of
light.
Why? What is the size of that circle?
Checkpoint 2
Electricity & Magnetism Lecture 25, Slide 16
A light ray travels in a medium with n1 and
completely reflects from the surface of a medium
with n2.
The critical angle depends on:
A. n1 only
B. n2 only
C.both n1 and n2
How much is reflected?
Unit 25, Slide 17
When going from n1>n2, some is reflected, some is transmitted
(refracted)
How much goes into each? Complex. We can do extremes.
o
i 90
2
12
12 )(nn
nnR
o
i 0
1R
Example 25.3: Reflection from Window
Topic 25, Slide 18
If you looked straight into a
window made of regular
glass, you would see a
reflection of the things behind
you.
What refraction of the
intensity of the original waves
would in that reflected
image?
Polarization
Topic 25, Slide 19
Unpolarized Intensity decreased
in plane of incidence
Partially polarized
plane of incidence
Polarization
Unit 25, Slide 20
112 cos)90sin(sin 9021
Snell’s Law: 111222 sincossin nnn
1
21tan
n
n
Completely polarized
when Q1 + Q2 = 90o
Brewster’s Angle:
QB = tan-1(n2/n1)
Example 25.4: Brewster’s Angle
Topic 25, Slide 21
A light beam is incident on a piece of glass
(n=1.5) at the minimum angle for which to
reflected beam is completely polarized.
What is Qr for the portion of the beam that
is refracted? QB
Qr
CheckPoint 4
Topic 25, Slide 22
Some of the light also reflects off the surface of the water. If the
incident light is initially unpolarized, the reflected light will be
A. Unpolarized
B. Somewhat horizontally polarized
C. Somewhat vertically polarized
A ray of light passes from air into water with an angle of incidence of
30 degrees.
30o
CheckPoint 3
Electricity & Magnetism Lecture 25, Slide 23
A light is shining at the bottom of a swimming pool
(shown in yellow in the figure). A person is standing at
the edge of the pool as shown.
Can the person standing on the edge of the pool be prevented from
seeing the light by total internal reflection at the water-air surface?
A. yes
B. no
Dispersion
Topic 25, Slide 24
Speed of E&M wave in media decreases in
media slightly with frequency
n is slight frequency (wavelength) dependent
dispersion
Rainbows
Topic 25, Slide 25
Mixed (white) light
Refraction (dispersion)
reflection
Refraction (dispersion)
Rain
drops
84o
50 cm
20 0 40 60 80 100
45o
A meter stick lies at the bottom of a rectangular water tank of height 50cm. You look into the tank at an angle of 45o relative to vertical along a line that skims the top edge of the tank.
nwater 1.33
Example 25.4: Meter Stick in Tank
Unit 25, Slide 26
What is the smallest number on the meter stick that
you can see?
meter stick
from n2 to n1
Summary
Unit 25, Slide 27