Lecture 7
Chapter 3
Electromagnetic Theory, Photons.
and Light
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Sources of light
Emission of light by atoms
The electromagnetic spectrum – see supplementary material
Light in bulk matter and dispersion
Sources of light
Accelerating charges emit light
Linearly accelerating charge
Synchrotron radiation—
light emitted by charged
particles deflected by a
magnetic field
Bremsstrahlung (Braking radiation)—
light emitted when charged particles
collide with other charged particles
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B
Synchrotron radiation:
Advanced Photon Source
Argonne National Lab, Chicago, IL
1104 m circumference storage ring
http://www.aps.anl.gov/
The vast majority of light in the universe
comes from molecular vibrations emitting light.
Electrons vibrate in their motion around nuclei
High frequency: ~1014 - 1017 cycles per second.
Nuclei in molecules vibrate
with respect to each other
Intermediate frequency:
~1011 - 1013 cycles per second.
Nuclei in molecules rotate
Low frequency: ~109 - 1010 cycles per second.
Emission of light by (isolated) atoms
Quantum mechanics: electrons in atoms can only be in discreet
states characterized with specific (quantized) energy
Transition of electron between discreet states with different
energies causes emission or absorption of a single photon with
energy matching the energy difference between the electron states
The energy of this photon and frequency of EM wave are
connected via Planck’s constant: E = h
Atomic and molecular vibrations
correspond to excited energy levels in
quantum mechanics.
Energy levels are everything in quantum mechanics.
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Energy
Excited level
E = h
Ground level
The atom is vibrating
at frequency, .
The atom is at least partially in
an excited state.
Excited atoms emit photons
spontaneously.
When an atom in an excited state falls to a lower energy level, it
emits a photon of light.
Energy
Excited level
Ground level
Molecules typically remain excited for no longer than a few
nanoseconds. This is often also called fluorescence or, when it
takes longer, phosphorescence.
Different atoms emit light at different
widely separated frequencies.
Each colored
emission line
corresponds to
a difference
between two
energy levels.
These are
emission
spectra from
gases of hot
atoms.
Frequency (energy)
Atoms have relatively simple energy level systems (and hence simple
spectra).
Atoms and molecules can also absorb
photons, making a transition from a lower
level to a more excited one.
Excited level
Energy
This is, of
course,
absorption.
Ground level
Absorption lines in an
otherwise continuous
light spectrum due to
a cold atomic gas in
front of a hot source.
Before
After
Spontaneous
emission
Absorption
Stimulated
emission
Einstein showed that stimulated emission can also occur.
Molecules have many energy levels.
A typical molecule’s energy levels:
E = Eelectonic + Evibrational + Erotational
2nd
excited
electronic state
Energy
1st excited
electronic state
Lowest vibrational and
rotational level of this
electronic “manifold”
Excited vibrational and
rotational level
Transition
Ground
electronic state
There are many other
complications, such as
spin-orbit coupling,
nuclear spin, etc.,
which split levels.
As a result, molecules generally have very complex spectra.
Water’s vibrations
Energy
Decay from an excited state can occur in
many steps.
Ultraviolet
Infra-red
Visible
Microwave
The light that’s eventually re-emitted after absorption may occur
at other colors.
The Greenhouse effect
The greenhouse effect occurs because
windows are transparent in the visible but
absorbing in the mid-IR, where most
materials re-emit. The same is true of the
atmosphere.
Visible
Infra-red
Greenhouse gases:
carbon dioxide
water vapor
methane
nitrous oxide
Methane, emitted by
microbes called
methanogens, kept
the early earth warm.
Blackbody radiation
Blackbody radiation is emitted from a hot body. It's anything but black!
The name comes from the assumption that the body absorbs at every
frequency and hence would look black at low temperature.
It results from a combination of spontaneous emission, stimulated
emission, and absorption occurring in a medium at a given
temperature.
It assumes that
the box is filled
with molecules
that, together,
have transitions
at every
wavelength.
Blackbody emission spectrum
The higher the temperature, the more the emission and the shorter
the average wavelength.
Blue hot is hotter
than red hot.
The sun’s surface is 6000 degrees K, so its blackbody spectrum
peaks at ~ 500 nm--in the green. However, blackbody spectra are
broad, so it contains red, yellow, and blue, too, and so looks white.
Electromagnetic spectrum
See supplementary lecture notes
Light in bulk matter
Maxwell eq-ns in free space  EM wave speed is c 
1
 0 0
In medium, 0 and 0 in Maxwell equation must be replaced by 
and  and phase speed of EM wave in medium becomes slower:
v
1

Absolute index of refraction: n 
Relative permittivity: K E    0
Relative permeability: K B   0
c


v
 0 0
n  KE KB
For nonmagnetic transparent materials KB1: n  K E
Maxwell’s
Relation
However, n depends on frequency (dispersion) and Maxwell
equation works only for simple gases.