Transcript Let There Be Light

Let There Be Light
13.3
Where are we?
• Exam #3 on Monday Morning
– Watch for another of those
Webby Things
• Today, we continue our study of
optics and continue working with
the optics kit.
• Next week … optics marathon.
Start reading
chapter 24
after the test
Which one of the following statements concerning electromagnetic waves
is false?
A) Electromagnetic waves are longitudinal waves.
B) Electromagnetic waves transfer energy through space.
C) The existence of electromagnetic waves was predicted by
Maxwell.
D) Electromagnetic waves can propagate through a material
substance.
E) Electromagnetic waves do not require a physical medium for
propagation.
The electric field E of an electromagnetic wave traveling the positive x
direction is illustrated in the figure. This is the wave of the radiation field of
an antenna. What are the direction and the phase relative to the electric field
of the magnetic field at a point where the electric field is in the negative y
direction?
Note: The wave is shown in a region of space that is a large distance from its
source.
A) +y direction, in phase
B) –z direction, 90° out of phase
C) +z direction, 90° out of phase
D)–z direction, in phase
E)+z direction, in phase
Think about the diagram we used yesterday and how
the charge moves to produce the wave in the first place!
Glare tends to be
A.
B.
C.
D.
un-polarized light
light polarized vertically
light polarized horizontally
the polarization depends on the
conductivity of the material causing the
glare
Which one of the following
colors of visible light has the
highest frequency?
A)
B)
C)
D)
E)
yellow
red
green
blue
violet
Like all waves, electromagnetic waves have a wavelength
and frequency, related by:
c  f
MEASURING THE SPEED OF LIGHT WITHOUT
ELECTRONICS … THE OLDEN DAYS
The speed of light
in a vacuum
c  299 792 458 m s
24.3 THE SPEED OF LIGHT
Maxwell’s prediction of the speed of light
c
1
 o o

8.85 10
12

1

C 2 N  m 2 4 10 7 T  m A

 3.00 108 m s
MORE ON POLARIZATION
FROM CAPACITOR AND COIL WE FOUND
The total energy density carried by an electromagnetic
wave
Total energy 1
1 2
2
u
 oE 
B
Volume
2
2o
Must contain moving energy
INTENSITY
P Energy 1
Total energy uctA
S 


 cu
A
time Area
tA
tA
E AND B ARE COUSINS!


Maxwell’s Equations show that the energy density of the
magnetic field and the electric field in an electromagnetic wave
are the same.
1
1 2
u  0E2 
B (Peak Values)
Consequently
2
2 0
so
u  0E2 
1
0
B2
since
c
1
 0 0
we get
E=cB
E rms 
E
2
E rms 
E
2
MORE ON POLARIZATION
In polarized light, the electric field
fluctuates along a single direction.
IMPORTANT STUFF HERE
Polarized light may be produced from unpolarized light with
the aid of polarizing material.
MALUS’ LAW
sin 2   cos 2   1
sin 2   cos 2 
2 cos 2   1
S  S o cos 
2
intensity after
analyzer
cos 2  
intensity before
analyzer
1
2
24.6 POLARIZATION
Example 7 Using Polarizers and Analyzers
What value of θ should be used so the average intensity of the polarized
light reaching the photocell is one-tenth the average intensity of the
unpolarized light?
S0
S0/2
1
10
S o  12 S o cos 2 
1
5
 cos 2 
cos  
1
5
  63.4
Conceptual Clicker
Suppose that a third piece of polarizing material is inserted
between the polarizer and analyzer. Does light now reach the
photocell?
A Yes
B No
C More info required.
THE EXPERIMENT - MIRRORS
Complete the last one first.
REFLECTION
Prepare to Discuss These Problems After The
Experiment:
1.A friend is standing 2 m in front of a plane mirror. You are standing 3 m
directly behind your friend.
2.How far is your image behind the mirror?
3.What is the distance between you and the image of your friend?
4.Suppose that you are walking perpendicularly with a velocity of +0.90 m/s
toward a stationary plane mirror. What is the velocity of your image relative to
you? The direction in which you walk is positive direction.
GET TO WORK ON MIRRORS
I Feel Horrible!