Electromagnetic waves
• Lecture topics
• Generation of EM waves
• Terminology
• Wave and particle models of EM radiation
• EM spectrum
Generation of EM waves
• Acceleration of an electrical charge
• EM wavelength depends on length of time that the charged
particle is accelerated
• Frequency depends on number of accelerations per second
• ‘Antennas’ of different sizes
• Nuclear disintegrations = gamma rays
• Atomic-scale antennas = UV, visible, IR radiation
• Centimeter/Meter-scale antennas = radio waves
• http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=35
Oscillating electric dipoles
Electric dipole: separation of
positive and negative charges
(permanent or induced)
Water molecule
• There is no fundamental constraint on the frequency of EM radiation,
provided an oscillator with the right natural frequency and/or an energy
source with the minimum required energy is present
Electromagnetic Spectrum
• EM Spectrum
• Continuous range of EM radiation
• From very short wavelengths (<300x10-9 m)
• High energy
• To very long wavelengths (cm, m, km)
• Low energy
• Energy is related to wavelength (and hence
frequency)
EM wave terminology
• EM waves characterized by:
• Wavelength,  (m)
• Amplitude, A (m)
• Velocity, v or c (m s-1)
• Frequency, f or ν (s-1 or
Hz) – cycles per second
• Sometimes period, T (time
for one oscillation i.e., 1/f)
v
Wavelength units
• EM wavelength  specified using various units
• cm (10-2 m)
• mm (10-3 m)
• micron or micrometer, m (10-6 m)
• nanometer, nm (10-9 m)
• Angstrom, Å (10-10 m, mostly used in astronomy)
• f (or ν) is waves/second, s-1 or Hertz (Hz) – also MHz, GHz
• Wavenumber (inverse wavelength) also commonly used:
given by 1/ (sometimes also 2π/λ) e.g. cm-1 (symbol: )
• What is the wavenumber (in cm-1) equivalent to  = 1 µm?
Electromagnetic energy
• EM radiation defined by wavelength (), frequency (f) and velocity (v) where:
v = f
• i.e. longer wavelengths have lower frequencies etc.
• v and  can change according to medium – f is constant
• Generally more useful to think in terms of  (numbers are easier)
• NB. Where v = c, this relationship refers to wavelength in a vacuum.
Digression – radio waves
• Why is FM radio higher quality than AM radio?
AM
FM
Digression – radio waves
• Why is FM radio higher quality than AM radio?
• AM = Amplitude modulation (530 – 1700 kHz)
• FM = Frequency modulation (87.8 – 108 MHz)
• EM wave amplitude can be affected by many things – passing
under a bridge, re-orienting the antenna etc.
• No natural processes change the frequency
• Radiation with frequency f will always have that frequency until it
is absorbed and converted into another form of energy
Wave phase and angular frequency
• Angular frequency ω = 2πf = 2π/T
• Frequency with which phase changes
• Angles in radians (rad)
• 360° = 2 rad, so 1 rad = 360/2
= 57.3°
• Rad to deg. (*180/) and deg. to
rad (* /180)
Light is not only a wave, but also a particle
Newton proposed wave theory of light (EMR) in 1666:
observation of light separating into spectrum
The Photoelectric Effect (H. Hertz [1887], A. Einstein
[1905]) – visible light incident on sodium metal
Posed problems if light was just a wave:
The electrons were emitted immediately (no time lag)
Increasing the intensity of the light source increased
the number of electrons emitted but not their energy
Red light did not cause any electrons to be emitted, at
any intensity
Weak violet light ejected fewer electrons, but with
greater energy
Max Planck (1900) found that electron energy was proportional to the frequency of the
incident light
Wave-particle duality
Property of EM radiation
Consistent with WAVE
PARTICLE
Reflection
Yes
Yes
Refraction
Yes
Yes
Interference
Yes
No
Diffraction
Yes
No
Polarization
Yes
No
Photoelectric effect
No
Yes
Photons
The energy of a single photon is: hf or
 = (h/2π)ω
where h is Planck's constant, 6.626 x 10-34 Joule-seconds
One photon of visible light contains about 10-19 Joules - not much
Φ is the photon flux, or
the number of photons
per unit time in a beam.
P P
 
hv hc
Where P is beam power.
Particle model of radiation
• EMR intimately related to atomic structure and energy
• Atom: +ve charged nucleus (protons+neutrons) & -ve
charged electrons bound in orbits
• Electron orbits are fixed at certain levels, each level
corresponding to a particular electron energy
• Change of orbit either requires energy (work done), or releases
energy
• Minimum energy required to move electron up a full energy level
(can’t have shift of 1/2 an energy level)
• Once shifted to a higher energy state from the ground state, the
atom is excited, and possesses potential energy
• Released as electron falls back to lower energy level
Particle model of radiation
Bohr quantized shell model of the atom (1913): electrons jump from one orbit to
another only by emitting or absorbing energy in fixed quanta (levels)
If an electron jumps one orbit closer to the nucleus, it must emit energy equal to the
difference of the energies of the two orbits. When the electron jumps to a larger orbit, it
must absorb a quantum of light equal in energy to the difference in orbits.
Particle model of radiation: atomic shells
Electron energy levels are unevenly spaced and characteristic of a
particular element. This is the basis of spectroscopy.
To be absorbed, the energy of a photon must match one of the
allowable energy levels in an atom or molecule.
Electromagnetic energy
• EM radiation also considered in quantum terms, where each
photon carries an energy E (in Joules) given by:
E = hf (or hν)
• where h is Planck’s constant (6.626x10-34 J s), f = frequency
Electromagnetic energy
• Combining the two relations we have:
E
hv

• i.e. the energy of a photon is inversely proportional to λ
• Implications for sensor design, pixel size etc.
Frequency decomposition
• Naturally occurring EM radiation hardly ever
consists of a single frequency or wavelength
• But, any arbitrary EM fluctuation can be thought
of as a composite of a number (potentially infinite)
of different ‘pure’ periodic functions
• This is known as Fourier decomposition
• So any EM wave can be regarded as a mixture
of pure sine waves with differing frequencies, and
the propagation of each frequency component can
be tracked completely separately from the others.
• In remote sensing, the implication is that
individual frequencies can be considered
individually, then the results summed over all
relevant frequencies.
Plane waves have only
one frequency, ω.
Light electric field
Broadband vs. Monochromatic
Time
This light wave has many
frequencies. And the
frequency increases in
time (from red to blue).
• EM radiation composed entirely of a single frequency is termed
monochromatic (‘one color’)
• Radiation that consists of a mixture of frequencies is called broadband.
• So transport of broadband radiation can always be understood in terms
of the transport of individual constituent frequencies (monochromatic
radiation)
Photochemistry
Many chemical reactions that take place in the atmosphere, including
those that produce smog, are driven by sunlight.
The stratospheric ozone layer also owes its existence to photochemical
processes that break down oxygen molecules (O2).
The photon energy E = hν is a
crucial factor in determining which
frequencies of EM radiation
participate in these processes.
Requires λ < 0.4 µm (i.e., sunlight)
Production of tropospheric ozone (a major pollutant)
The Electromagnetic Spectrum
The EM spectrum is subdivided into a few
discrete spectral bands.
EM radiation spans an enormous range of
frequencies; the bands shown here are
those most often used for remote sensing.
Boundaries between bands are arbitrary
and have no physical significance, except
for the visible band.
Note that the ‘visible’ band is subjective –
some insects can see ultraviolet light!
What wavelengths are associated with sunburn?
The Ultraviolet (UV)
The UV is usually broken up into three regions, UV-A (320-400
nm), UV-B (290-320 nm), and UV-C (220-290 nm).
UV-C is almost completely absorbed by the atmosphere. You can
get skin cancer even from UV-A.
Remote sensing of ozone (O3) uses UV radiation.
Photodissociates O2
and O3; absorbed
between 30 and 60 km
Mostly absorbed by O3 in stratosphere; small
fraction (0.31-0.32 µm) reaches surface and
causes sunburn (effect of ozone depletion?);
energetic enough for photochemistry
Reaches surface;
Relatively harmless;
Stimulates fluorescence in
some materials
Visible light
Wavelengths and frequencies
of visible light (VIS)
Atmosphere mostly transparent – optical remote
sensing techniques, surface mapping etc.
• Includes wavelength of peak
emission of radiation by the Sun
(~50% of solar output in this
range)
• Cloud-free atmosphere mostly
transparent to VIS wavelengths,
so most are absorbed at the
Earth’s surface
• Clouds are highly reflective in
the VIS – implications for
climate?
The Infrared (IR)
• Sub-mm wavelengths
• Unimportant for atmospheric
photochemistry – why?
• IR regions subdivided by wavelength
and/or source of radiation
– Region just longer than visible known
as near-IR, NIR (0.7 – 4 µm) - partially
absorbed, mainly by water vapor
– Reflective (shortwave IR, SWIR)
– Emissive or thermal IR (TIR; 4 – 50
µm) – absorbed and emitted by water
vapor, carbon dioxide, ozone and other
trace gases; important for remote sensing
and climate
Note boundary (~4 µm) – separates
shortwave and longwave radiation
– Far IR (0.05 – 1 mm) – absorbed by
water vapor; not widely exploited
The microwave (µ-wave) region
• RADAR
• mm to cm wavelengths
• Usually specified as frequency,
not wavelength
• Various bands used by RADAR
instruments
• Long  so low energy, hence
require own energy source
(active microwave)
• Penetrates clouds, planetary
atmospheres – useful for
mapping
• Weather – monitor rainfall,
tornadoes, t-storms etc.
The electromagnetic spectrum
The transition wavelengths are a bit arbitrary…
Now, we’ll run through the entire electromagnetic spectrum, starting at
very low frequencies and ending with the highest-frequency gamma rays.
60-Hz radiation from
power lines
This very-low-frequency current
emits 60-Hz electromagnetic waves.
No, it is not harmful. A flawed epidemiological study in 1979 claimed
otherwise, but no other study has
ever found such results.
Also, electrical power generation has increased exponentially
since 1900; cancer incidence has remained essentially constant.
Also, the 60-Hz electrical fields reaching the body are small;
they’re greatly reduced inside the body because it’s conducting;
and the body’s own electrical fields (nerve impulses) are much
greater.
Long-wavelength
EM spectrum
Arecibo radio
telescope
Radio & microwave regions (3 kHz – 300 GHz)
Global positioning system (GPS)
• Consists of 24 orbiting satellites in “half-synchronous orbits” (two
revolutions per day).
• Four satellites per orbit,
equally spaced, inclined
at 55 degrees to equator.
• Operates at 1.575 GHz
(1.228 GHz is a reference
to compensate for atmospheric water effects)
• 4 signals are required;
one for time, three for
position.
• 2-m accuracy
Microwave ovens
Microwave ovens operate at 2.45 GHz,
where water absorbs very well.
Percy LeBaron
Spencer, Inventor
of the microwave
oven
Geosynchronous communications satellites
22,300 miles (36,000 km) above the earth’s surface
6 GHz uplink, 4 GHz downlink
Each satellite is actually two (one is a spare)
Cosmic
microwave
background
Microwave background vs. angle
Peak frequency is ~ 150 GHz
The cosmic microwave
background is blackbody
radiation left over from
the Big Bang
Wavenumber (cm-1)
• Interestingly,
blackbody radiation
retains a blackbody
spectrum despite
the expansion of the
universe. It does get
colder, however.
IR is useful for
measuring the
temperature of
objects.
Hotter and
hence brighter
in the IR
Old Faithful
Infrared
Lie-detection
The military uses IR to see objects it
considers relevant
IR light penetrates fog and smoke better than visible light.
The infrared space observatory
Stars that are just
forming emit light
mainly in the IR.
Using mid-IR laser light
to shoot down missiles
Wavelength =
3.6 to 4.2 m
The Tactical High Energy Laser uses a high-energy,
deuterium fluoride chemical laser to shoot down
short range unguided (ballistic flying) rockets.
Laser welding
Near-IR wavelengths are
commonly used.
Laser pointer (red)
Auroras
Solar wind particles spiral around the earth’s
magnetic field lines and collide with atmospheric molecules, electronically exciting them.
Auroras are due to
fluorescence from
molecules excited by
these charged particles.
Different colors are from
different atoms and
molecules.
O: 558, 630, 636 nm
N2+: 391, 428 nm
H: 486, 656 nm
Fluorescent lights
Use phosphors (transition metal compounds that exhibit phosporescence
when exposed to UV light)
“Incandescent” lights (normal light bulbs) lack the emission lines
The eye’s response to light and color
The eye’s cones have three receptors, one for
red, another for green, and a third for blue.
The eye is poor at distinguishing spectra
Because the eye perceives intermediate colors, such as orange and
yellow, by comparing relative responses of two or more different
receptors, the eye cannot distinguish between many spectra.
The various yellow spectra below appear the same (yellow), and the
combination of red and green also looks yellow
UV from the sun
The ozone layer absorbs wavelengths less than 320 nm (UV-B and
UV-C), and clouds scatter what isn’t absorbed.
But much UV (mostly UV-A, but some UV-B) penetrates the
atmosphere anyway.
IR, Visible, and UV Light and Humans
(Sunburn)
Skin
surface
We’re opaque in the UV and visible, but not necessarily in the IR.
Flowers in the UV
Since bees see in the UV (they have a receptor peaking at 345 nm),
flowers often have UV patterns that are invisible in the visible.
Arnica angustifolia Vahl
Visible
UV (false color)
The sun in the UV
Image taken
through a
171-nm filter
by NASA’s
SOHO
satellite.
The very short-wavelength regions
Vacuum-ultraviolet (VUV)
180 nm > > 50 nm
Soft x-rays
5 nm > > 0.5 nm
Absorbed by <<1 mm of air
Ionizing to many materials
Strongly interacts with core
electrons in materials
Extreme-ultraviolet (XUV or EUV)
50 nm > > 5 nm
Ionizing radiation to all materials
EUV Astronomy
The solar corona is very hot (30,000,000 degrees K) and so emits
light in the EUV region.
EUV astronomy requires satellites because the earth’s atmosphere is
highly absorbing at these wavelengths.
The sun also emits x-rays
The sun seen in the x-ray region
Matter falling into a black hole emits x-rays
Nearby star
Black hole
A black hole accelerates particles to very high speeds
Supernovas emit x-rays, even afterward
A supernova
remnant in a
nearby galaxy (the
Small Magellanic
Cloud).
The false colors
show what this
supernova
remnant looks like
in the x-ray (blue),
visible (green) and
radio (red) regions.
X-rays are occasionally seen in auroras
On April 7th 1997, a
massive solar storm
ejected a cloud of
energetic particles
toward planet Earth.
The “plasma cloud” grazed the Earth,
and its high energy particles created a
massive geomagnetic storm.
Atomic structure and x-rays
Ionization energy
~ 100 – 1000 e.v.
Ionization energy
~ .01 – 1 e.v.
X-rays penetrate tissue and do not
scatter much
Roentgen’s x-ray image
of his wife’s hand (and
wedding ring)
X-rays for photo-lithography
You can only focus light to
a spot size of the light
wavelength. So x-rays are
necessary for integratedcircuit applications with
structure a small fraction
of a micron.
1 keV photons from a
synchrotron:
2 micron lines over a base
of 0.5 micron lines.
Gamma rays result from matterantimatter annihilation
An electron and positron self-annihilate, creating two gamma
rays whose energy is equal to the electron mass energy, mec2.
e-
e+
h = 511 kev
More massive particles create even more energetic gamma
rays. Gamma rays are also created in nuclear decay, nuclear
reactions and explosions, pulsars, black holes, and
supernova explosions.
Gamma-ray bursts emit massive
amounts of gamma rays
A new one
appears almost
every day, and
it persists for
~1 second to
~1 minute.
The gamma-ray sky
They’re
probably
supernovas.
In 10 seconds, they can emit more energy than our sun will in its
entire lifetime. Fortunately, there don’t seem to be any in our galaxy.
The universe in
different spectral
regions…
Gamma Ray
X-Ray
Visible
The universe in more spectral
regions…
IR
Microwave