Transcript EM Waves history & Polarization APIB

Wave Nature of Light &
Electromagnetic Waves
History, Light is a Wave &
Polarization
Terms
Superposition: Algebraic sum of the
displacements of waves.
 Interference: Pattern produced by
interfering waves, can be constructive or
destructive.
 Diffraction: The spreading of light beyond
a barrier. (Huygens principle applies)

Conditions of Interference
Best if monochromatic – single frequency,
one color light
 Coherent – Constant phase relationship.
Same frequency and in phase. (variance
of one’s phase to the other is constant)
 Zero Phase difference “In Phase” –
Constructive
 180˚ difference in phase – out of phase
by ½ λ so it is destructive

Rainbows
Chromatic Dispersion
 Index of refraction is greater for a shorter
wavelength (blue) than for a longer
wavelength (red)
 Occurs with prisms where white light is
spread into its constituant colors.
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Christian Huygens 1678

Every point of a wave front may be
considered the source of secondary
wavelets that spread out in all
directions with a speed equal to the
speed of propagation of the waves.
Thomas Young, 1801
Experiment demonstrated the wave nature
of light and
 Allows one to determine the wavelength, λ
of a beam of light
 Measured the average wavelength of
sunlight. (570 nm measured – Current is
550 nm. Very close!)

Newton was wrong
If light had been particles or corpuscles as
Newton considered them, then it would
pass directly through the slits to the
screen beyond in Young’s experiment.
 This did not happen!

Phase difference can change…

The phase difference between two waves
can change if the waves travel paths of
different lengths.
Path Length Differences
Equal to mλ - constructive & bright fringe
 Equal to ½mλ - destructive and dark fringe.

Interference from path difference
If d << L then the difference in path length
travelled by the two rays is approximately:
r1 - r2 ≈ dsin θ
 Where θ is approximately equal to the
angle that the rays make relative to a
perpendicular line joining the slits to the
screen.

Bright & Dark Fringes
If the rays were in phase when they passed
through the slits, then the condition for
constructive interference at the screen is:
dsinθ = m λ ,m = ±1, ±2,...
 The condition for destructive interference at
the screen: dsinθ= (m + ½)λ ,m = ±1, ±2,...


Where m is the order of the interference fringe.
(m=1 is on either side of the central fringe)
When y << L
sin θ ≈ y/L (Your text uses x instead of y)
 So y = λmL/d

Diffraction Grating
Large number of equally spaced parallel
slits.
 Creates even sharper and narrower bright
maxima than Young’s Double Slit.
 Useful for precise measurements of
wavelengths.

Thin Film Interference

Interference of light waves reflecting off
the top surface of a film with the waves
reflecting from the bottom surface
Examples of Thin Film Interference

CD

Oil

Soap Bubble
Electromagnetism
James Clerk Maxwell 1864
Hypothesized that since a changing magnetic
field produces an electric field (Faraday’s Law)
the opposite could be true.
 Worked out mathematically these described:

Charge Density and the Electric Field
The Structure of the Magnetic Field
A Changing Magnetic Field and the Electric Field
The Source of the Magnetic Field
Leading to…
Electric and magnetic fields acting together
could produce an electromagnetic wave which
travels at the speed of light.
 Prior to this, visible light was thought of as a
completely separate phenomenon.
 Now light is understood to be an
electromagnetic wave
 His theory also implied that electromagnetic
waves were not limited to visible light.

Heinrich Hertz, 1887
First produced and observed EM waves in
a laboratory setting using LC circuit.
 Observed energy transfer
 Observed normal wave behavior such as
reflection, refraction, interference,
diffraction and polarization.

Hertz’s Experiment Showed
Wave was produced in one circuit and
propagated across the room to a second
circuit.
 The wave moved at the speed of light!

Leading us to Marconi
Guglielmo Marconi 1896 recognized the
implications of the EM wave experiments.
 Waves could be used for communications,
eliminating the need for telegraph wires.
 Radio Signal

 Sent
from Cornwall England
 Received in St. Johns Newfoundland
How does a turtle find a new pond?
Polarization
 In general: The polarization of an EM
wave refers to the direction of its electric
field.
 Light from the sun or an incandescent
bulb is unpolarized.

How is light polarized…
Pass a beam of light through a polarizer.
 Polarizer is a material that is composed of
long thin electrically conductive molecules
oriented in a specific direction.
 Reflected light is polarized

Polarizer
When unpolarized light hits a single layer
of polarizing material ½ the light goes
through.
 If a second sheet is at a right angle to the
first, no light will transmit through the
second sheet.
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Sources
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http://www.rare-earthmagnets.com/magnet_university/maxwells_equations.htm
Physics, 4th Edition, Walker, James S.; Pearson Education, 2010.
http://physics.about.com/od/mathematicsofwaves/a/huygensprincipl.htm
http://www.mathpages.com/home/kmath242/kmath242.htm
http://micro.magnet.fsu.edu/primer/java/interference/doubleslit/doubleslitja
vafigure1.jpg
http://www.daviddarling.info/images/Youngs_double-slit_experiment.jpg
http://theory.uwinnipeg.ca/physics/light/node9.html
electron9.phys.utk.edu
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