Physics 123C Waves and Modern Physics

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Physics 123C Waves and Modern Physics
Lecture 8 (TM 31.4-7)Polarization,
Spectra October 16, 2009 (20 Slides)

• John G. Cramer
• Professor of Physics
• B451 PAB
• cramer_at_phys.washington.edu

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Lecture 8 Announcements

• So far 171/196 students have registered their
  HiTT Clickers. You may obtain a Clicker from the
  University Book Store and should register it
  (using the link on the 123A Syllabus page) as
  soon as possible.
• Lecture Homework 3 has been posted on the
  WebAssign system. It is due at 1159 PM on
  Wednesday, October 21.
• We will have Exam 1 in this room on Wednesday,
  October 21. It will consists of a 60 point
  multiple-choice part and two 20 point
  free-response parts, one based on WebAssign
  homework and one based on the tutorial. You may
  bring one sheet of notes (both sides).
• We will have a pre-exam review on Monday.
  Bring questions about difficult points in the
  material covered.

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Lecture Schedule (Part 1)
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Wave Modes on a String
Since the displacement of waves on a string
is transverse, they can be produced in either of
two directions (vertical or horizontal).
These are two distinct wave modes that have the
same frequency, wavelength, and propagation
speed, but may have different and unrelated
amplitudes and phases.
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Polarization of Light
Since EM waves are transverse, for a given
direction of propagation there are two
perpendicular directions in which the electric
field can oscillate. The E field and Poynting
vector S define the plane of polarization of the
wave. The figures show examples of vertical (a)
and horizontal (b) polarization. Light
reflected from a surface as glare tends to have
its plane of polarization parallel to the
reflecting surface.
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Polarization Maluss Law
A polarizing filter blocks the component of
light that has the plane of polarization in a
particular direction, i.e., it absorbs and blocks
that component of the E field. Unpolarized light
passed through such a filter emerges polarized in
the pass direction only, with about ½ the
intensity of the incident beam. If such a
polarized wave now is passed through a second
polarizing filter, the intensity of the
transmitted wave depends of the angle q between
the filter pass direction and the incident plane
of polarization. The relation for intensity is
called Maluss Law
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ExampleIntensity Transmitted
Unpolarized light of intensity 3.0 W/m2 is
incident on two polarizing sheets whose
transmission axes make an angle of 60. What is
the intensity of light transmitted by the second
sheet?
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Clicker Question 1
Unpolarized light of equal intensity is
incident on each of the four pairs of polarizing
filters shown below. Which system will
transmit the maximum intensity?
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Polarization by Reflection
When unpolarized light is reflected from a
plane surface boundary between two transparent
media, e.g., air/glass or air/water, the
reflected light is partially polarized. The
degree of polarization depends on the angle of
incidence and the indices of refraction in the
two media. For a certain angle of incidence
called the polarizing angle qp, the reflected
light is completely polarized. At qp, the
reflected and refracted rays are perpendicular to
each other. David Brewster (17811861)
discovered this experimentally in 1812. The
polarizing angle qp is also referred to as the
Brewster angle.
When the incident light is vertically polarizes,
there is no reflection.
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The Sky and Glare are Polarized
The light from the blue sky, which is
actually scattered sunlight, is polarized
perpendicular to the scattering plane. The
compound eyes of bees are sensitive to this sky
polarization, and they use it to navigate.
Light is preferentially reflected by a surface
when the electric field is parallel to the
reflecting plane. Therefore, road glare tends
to be polarized horizontally. Vertically
polarizing sunglasses suppress this glare.
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Polarization by Birefringence
Birefringence occurs in certain crystals and
plastics, where the speed of light depends on the
polarization and on the direction of propagation
of the light. When a light ray is incident on
such materials, it may be separated into two rays
called the ordinary ray and the extraordinary
ray. These rays are polarized in mutually
perpendicular directions, and they travel with
different speeds. Depending on the relative
orientation between the material and the incident
light beam, the two rays may also travel in
different directions. The optic axis is
defined as the axis along which the two rays
travel at the same speed on the same path.
If light is incident on a birefringent plate
perpendicular to its crystal face and
perpendicular to the optic axis, the two rays
travel in the same direction but at different
speeds. In a quarter-wave plate, the thickness
is such that a 90 phase difference will develop
between the waves of a particular wavelength as
they emerge.
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Circular Polarization
Using a quarter-wave plate, one can add
vertically and horizontally polarized light such
that the electric fields peak 900 out of phase.
The result is an electric field that corkscrews
through space with either a right-handed or
left-handed twist. This is called circular
polarization., with the two states called right
circular polarization and left circular
polarization. Circular polarization has the
interestingproperty that on reflection, it
reverses direction. For example, a beam of right
circular polarized light on reflection becomes
left circular polarized light.
QWP
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Fermats Principle and Reflection and Refraction
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Huygens Principle and Reflection and Refraction
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Photons Wave-Particle Duality
The work of Max Planck and Albert Einstein
around 1900-1905 showed that the energy of light
is quantized into small chunks or quanta of
energy. In other words, a beam of light can be
considered as a stream of particles called
photons, each photon traveling at the speed of
light and carrying one quantum Eg hf of
energy.
h is called Plancks Constant
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Atomic Emission of Light
In the positive column glow of the gas
discharge tube, the color depends on the type of
gas used. This observation in the late 1800s
stimulated interest in its use for chemical
identification. Fortunately, at the same time
developments in the diffraction and interference
of light made possible the grating spectrograph,
which could analyze light into wavelengths with
high precision. These careful spectrograph
measurements showed experimentally that(1) each
gas discharge emits light at discrete wavelengths
(called lines), and(2) the line spectrum of each
element in the periodic table is unique.
Theoretical Question Why?
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Spectroscopy
Dense hot self-luminous objects like the Sun
or a light bulb emit a continuous spectrum of
visible wavelengths. In contract, light
emitted in a low-pressure gas discharge
containsonly discrete individualwavelengths,
adiscrete spectrum. Thesespectral
linescan berecorded andmeasuredusing
agratingspectroscope.
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Fraunhofer Absorption Lines
In 1813, Joseph Fraunhofer, a glazier and
telescope maker of Munich, discovered that there
were dark lines in the spectrum of sunlight. He
carefully described and catalogued some 574 lines
and assigned the stronger ones letters A-K,
designations that are still used by astronomers.
The Fraunhofer lines are absorption lines
caused by sunlight passing through the cooler and
less dense gas surrounding the Sun, and provides
a direct indication of the elemental composition
of the Sun (and other stars).
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Absorption Spectra
Materials not only emit light, but they can
also absorb light. If white light is passed
through a gas as shown, the absorption in the gas
removes certain wavelengths, so that the
spectrograph shows an absorption spectrum, a
spectrum of white light with certain wavelengths
missing. Comparison of the emission and
absorption spectra of the same element show a
provocative difference. Some of the lines are
present in both, but the emission spectrum
contains some lines missing in the absorption
spectrum. The classical physics of the 1800s
was unable to account for either the spectral
lines or the differences in emission vs.
absorption.
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End of Lecture 8

• Before the next lecture on Friday, October 23,
  read TM, Chapters 31.1-3.
• We will have Exam 1 on Wednesday, October 21.
  It will have a 60 point multiple choice part and
  two 20 point free-response parts. We will have a
  pre-exam review on Monday. Bring questions.
• Lecture Homework 3 has been posted on the
  WebAssign system. It is due at 1159 PM on
  Wednesday, October 21.