W14D1:%20EM%20Waves,%20Dipole%20Radiation,%20Polarization%20and%20Interference

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W14D1EM Waves, Dipole Radiation,Polarization
and Interference
Todays Reading Course Notes Sections 13.8,
13.10, 14.1-14.3
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Announcements
Math Review Week 14 Tuesday 9-11 pm in 26-152 PS
10 due Week 14 Tuesday at 9 pm in boxes outside
32-082 or 26-152 Next Reading Assignment W14D2
Course Notes Sections 14.4-14.9
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Outline

• Generating Plane EM Waves
• Generating Electric Dipole EM Waves
• Microwaves
• Polarization
• Interference

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History

• Maxwells Equations 1865
• Predicted that light was an electromagnetic wave,
  but no way to prove this experimentally. No
  general acceptance of his theory
• Hertz 1888
• Figured out how to generate electromagnetic waves
  exactly the way we do it in class today. All of
  a sudden, Maxwell was golden

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History

• Hertz 1888
• There will never be any practical use for my
  discovery. It is a laboratory curiosity
• Marconi 1894
• Practical wireless telegraphy, commercial
  success

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Generating Plane EM Waves
First, how do you generate waves on a string and
where does the energy carried away by the wave
come from?
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DemonstrationVibrating Rubber Tube (hand
driven) You Do Work Pulling the String Down
Against Tension (Restoring Force)The Work You
Do Appears in theEnergy Radiated Away By Wave
http//tsgphysics.mit.edu/front/?pagedemo.phplet
numC 35show0
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Generating Plane EM Waves
You can generate EM waves in an analogous way (to
the string) by shaking the field lines(strings)
attached to charges.
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Shaking a Sheet of Charge
Students go to this applet, observe for a bit,
then UNCHECK Motion On box and generate some EM
waves by left clicking on silver ball and moving
mouse
http//peter-edx.99k.org/PlaneWave.html
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How to Think About Radiation E-Field

• E-Field lines like strings tied to plane of charge

This is the static field
This is the radiation field
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Concept Q. Generating Plane Waves
When you are pulling the charged plane down, the
radiation electric field right at the position of
the plane of charge is

1. up
2. down
3. zero
4. cannot tell, depends on past history

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Concept Q. Ans Generating Plane Waves
When you are pulling the charged plane down, the
radiation electric field right at the position of
the plane of charge is

• Up
• The radiation electric field right at the sheet
  resists you pulling the charged sheet down, just
  like tension in a string.
• The work you do overcoming that resistance is
  the source of the energy radiated away by the
  wave.

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Generating Electric Dipole EM Waves
In the real world there are no infinite planes of
charge. The radiation pattern from shaking just
one charge is as follows
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Generating Electric Dipole Radiation Applet
http//web.mit.edu/viz/EM/simulations/radiationcha
rge.jnlp
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Concept Q. Generating Plane Waves
The point charge below got a kick a little before
the moment shown. The direction of the kick was

1. Up or down
2. Left or right
3. Cannot tell, depends on past history

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Concept Q. Ans Generating Plane Waves
The point charge below got a kick a little before
the moment shown. The direction of the kick was

• Left or right
• When you move the charge left or right, it does
  not put a kink in the horizontal field lines, and
  that is what we observe above.

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State of Polarization
Describes how the direction of the electric field
in an EM wave changes at a point in space.

1. Linear polarization
2. Circular polarization
3. Elliptical polarization

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Lecture DemonstrationPolarization of Microwaves
K3
Some materials can absorb waves with the electric
field aligned in a particular direction (for
example, sunglasses)
http//tsgphysics.mit.edu/front/?pagedemo.phplet
numK 3show0
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Lecture Demonstration Polarization of Radio
Waves Dipole Antenna K4
http//tsgphysics.mit.edu/front/?pagedemo.phplet
numK 4show0
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Spark Gap GeneratorAn LC OscillatorThis is
what Hertz did in 1886
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Our spark gap antenna

• Oscillation after
• breakdown! (LC)

1) Charging time scale (RC)
3) Repeat
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Spark Gap Antenna
Accelerated charges are the source of EM waves.
Most common example Electric Dipole Radiation.
t 0
t T/4
t T/2
t T
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Spark Gap Antenna
http//web.mit.edu/viz/EM/movies/light/hiResAntenn
a.avi http//youtu.be/SV4kTSbFWRc
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Experiment 5Spark Gap GeneratorFind the
Angular Distribution of Radiation, and its
Polarization
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Interference
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Interference The difference between waves and
particles
No Interference if light were madeup of
particles
Interference If light is a wave we see
spreading and addition and subtraction
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Interference
Interference Combination of two or more waves to
form composite wave use superposition
principle. Waves can add constructively or
destructively

• Conditions for interference
• Coherence the sources must maintain a constant
  phase with respect to each other
• Monochromaticity the sources consist of waves of
  a single wavelength

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Interference Phase Shift
Consider two traveling waves, moving through
space
Look here as function of time
In phase
Constructive Interference
Phase shift
Look here as function of time
Destructive Interference
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Interference Phase Shift

• What can introduce a phase shift?
• From different, out of phase sources
• Sources in phase, but travel different distances
  because they come from different locations

constructive
destructive
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Extra Path Length
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Extra Path Length
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Phase Shift Extra Path?
What is exact relationship between extra path
length and phase shift?
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DemonstrationMicrowave InterferenceTwo
Transmitters
http//tsgphysics.mit.edu/front/?pagedemo.phplet
numP 4show0
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Microwave Interference

http//youtu.be/-O8V2QHkaLI http//web.mit.edu/viz
/EM/movies/light/distant.avi
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Microwave Interference

http//youtu.be/SkEdqP86hmU http//web.mit.edu/v
iz/EM/movies/light/close.avi
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Two In-Phase Sources Geometry
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Interference for Two Sources in Phase
Constructive
Destructive
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Concept QuestionTwo Slits with Width
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Concept Question Double Slit
Coherent monochromatic plane waves impinge on two
apertures separated by a distance d. An
approximate formula for the path length
difference between the two rays shown is

1. d sin ?
2. L sin ?
3. d cos ?
4. L cos ?

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Concept Q. Answer Double Slit
Answer 1. Extra path length d sin ?
The difference between the two paths can be seen
to have this value by geometrical construction
(using the triangle shown in yellow).
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Group Problem Lecture Demo
The distance to the interference minima are given
by
When L 1.16 m and d 0.24 m, suppose the
distance to the first minimum is measured to be
7.25 cm. What is the wavelength and frequency of
the microwaves?
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The Light EquivalentTwo Slits
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Lecture DemonstrationDouble Slit
http//tsgphysics.mit.edu/front/?pagedemo.phplet
numP 10show0
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Measure 1/10,000 of a Cm
Question How do you measure the wavelength of
light? Answer Do the same experiment we did
above with microwaves, but now with light!
Light wavelength is smaller by 10,000 times
compared to microwave But d can be smaller (0.1
mm instead of 0.24 m) So y will only be 10 times
smaller then the above experiment still
measurable
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Youngs Double-Slit Experiment
Bright Fringes Constructive interference Dark
Fringes Destructive interference
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Concept Q. Two Slit Interference
A B
In the two 2-slit interference patterns above, is
the frequency of the wave on the left (A) is
larger or smaller than the frequency of the wave
on the right (B)? The slit spacing d is the same
in both cases.

1. Frequency in A is larger than in frequency B
2. Frequency in A is smaller than infrequency B
3. Frequency in A is equal to frequency in B

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Con. Q. Answer Two Slit Interference
Answer 2. Frequency in A is smaller than in B
A B
Two ways to see this First By eye,
Second so the smaller
in B means smaller wavelength and thus
higher frequency.
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