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Electromagnetic spectrum
Radio waves
 > 30 cm
Used primarily for communication
AC current in wires: 60 Hz, emits constant EM wave background (=5000 km)
Energies of radio wave ‘photons’ are very low and quantized
nature of EM radiation at these frequencies is not noticeable.
Microwaves
 ~ 300 m - 30 cm
Radars
Cell phones,
Satellite TV
Outer space: neutral hydrogen emits 21 cm microwaves
Molecular state transitions - rotational and vibrational
Cesium clock: two closely spaced levels, ~ 9.2 GHz
Microwave oven
Microwave oven
~2.45 GHz (12.2 cm):
resonance for rotating
water molecules
(Oven is set slightly off
resonance for more
even heating)
Big bang and microwaves
1960s: Arno Penzias and Robert Wilson discovered that the
whole universe is full of microwave radiation, that was similar to
radiation of a body at 2.7 K (about -270°C).
This cosmic background radiation is a leftover of the ‘Big
Bang’ - the origin of the universe
(Nobel Prize 1978)
Infrared (IR) light
=780 nm - 1 mm
Warm matter emits broad band of infrared light - at higher temperatures
band shifts to shorter wavelength. Can use for IR imaging.
IR image
IR movie
www.footagehouse.com
CO2 laser (~20m)
cutting metal
Visible light
=400 nm - 700 mm
Sun spectrum:
Sources of light:
Sun, light bulb, fire
Most objects do not
emit light but reflect or
scatter light emitted by
other objects
Mixture of all wavelengths
is percepted by people as
‘white’ light.
Ultraviolet (UV)
=100 nm - 350 mm
Photon energy is high and it is readily absorbed by electronic
transitions in atoms and molecules, and could be enough to ionize
molecules - remove an electron from a molecule (photoelectric
effect).
UV is damaging to living tissue
X-Rays: discovery
=0.1 Å - 10 nm
X-rays - discovered by Röntgen in 1895
The first X-Ray by Röntgen:
the hand a famous anatomist Kölliker
photographic plate
X-Rays can penetrate through matter
Late 1895, John Winthrop Wright, MD, used X-Ray of a
patient’s swollen hand to locate 22-caliber bullet
induction coil
Crookes tube
X-Rays: crystal structures
The wavelength of X-rays is comparable with inter-atomic
distance (~1 Å) and they are used to determine structures of
periodic structures (crystals)
Structures of proteins are determined by X-rays
Structure of the cytochrome b6f
complex determined by means of
X-rays by Dr. W.A. Cramer’s
group (Biological Sciences,
Purdue University)
Gamma-Rays
 << 1 Å
Due to extremely short wavelength wave-properties of gamma
rays are hard to observe.
Gamma rays are emitted by nuclear transitions (nuclear bomb)
The Electromagnetic Spectrum:
More Fun Things
•
Sources of light: gases, liquids, and solids
•
•
•
Boltzmann's Law
Blackbody radiation
The electromagnetic spectrum
•
Long-wavelength sources
• and applications
•
Visible light and the eye
Courtesy Prof. Rick Trebino,
Georgia Tech
• Short-wavelength sources and applications
The electromagnetic spectrum
gamma-ray
microwave
1
106
visible
radio
infrared
0
105
41
10
3
10
UV
2
10
wavelength (nm)
The transition wavelengths are a bit arbitrary…
X-ray
1
10
0
10
-1
10
60-Hz radiation from
power lines
Yes, 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.
60-Hz magnetic fields inside the body are < 0.002 Gauss; the
earth’s magnetic field is ~ 0.4 G.
The longwavelength
electromagnetic
spectrum
Arecibo radio
telescope
Radio & microwave regions (3 kHz – 300 GHz)
Global positioning system (GPS)
It 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
(100 m for us).
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 above the earth’s surface
6 GHz uplink, 4 GHz downlink
Each satellite is actually two (one is a spare)
Cosmic
microwave
background
Peak frequency is ~ 150 GHz
The 3° cosmic microwave
background is blackbody
radiation left over from
the Big Bang!
Wavenumber (cm-1)
Microwave background
vs. angle. Note the
variations.
Interestingly,
blackbody radiation
retains a blackbody
spectrum despite
the expansion the
universe. It does
get colder, however.
TeraHertz light (a region of microwaves)
TeraHertz light is light with a frequency of ~1 THz, that is, with a
wavelength of ~300 m.
THz light is heavily absorbed by water, but clothes are transparent
in this wavelength range.
IR is useful for
measuring the
temperature of
objects.
Hotter and
hence brighter
in the IR
Old Faithful
Such studies help to confirm that Old
Faithful is in fact faithful and whether
human existence is interfering with it.
IR Liedetection
I don’t really buy
this, but I thought
you’d enjoy it…
He’s really sweating now…
The military uses IR to see objects it
considers relevant.
IR light penetrates fog and smoke better than visible light.
Jet engines emit infrared light from 3 to 5.5 µm
Energy
 (m)
This light is easily distinguished from the ambient infrared, which peaks
near 10 m and is relatively weak in this range
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.
Atmospheric penetration depth (from
space) vs. wavelength
Visible light
Wavelengths and
frequencies of visible
light
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
Dye lasers cover the entire visible spectrum.
Fluorescent lights
“Incandescent” lights (normal light bulbs) lack the emission lines.
The human retina
Rods
Cones
The retina is a mosaic of two basic types of photoreceptors, rods,
and cones.
Cones are highly concentrated in a region near the center of the
retina called the fovea. The maximum concentration of cones is
roughly 180,000 per mm2 there and the density decreases rapidly
outside of the fovea to less than 5,000 per mm2. Note the blind spot
caused by the optic nerve, which is void of any photoreceptors.
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!
How film and digital cameras work
Most digital cameras interleave
different-color filters
Color wheels
Hue = wavelength
Saturation = spectral width
Value = brightness (intensity)
The Ultraviolet
The UV is usually broken up into three regions, UVA (320-400
nm), UVB (290-320 nm), and UVC (220-290 nm).
UVC is almost completely absorbed by the atmosphere.
You can get skin cancer even from UVA.
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
Synchrotron Radiation
Formerly considered a nuisance to accelerators, it’s now often the
desired product!
Synchrotron radiation in all
directions around the circle
Synchrotron radiation only
in eight preferred directions
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.
Fast electrons
impacting a
metal generate
x-rays.
High voltage
accelerates electrons
to high velocity, which
then impact a metal.
Electrons displace electrons in
the metal, which then emit xrays.
The faster the electrons, the
higher the x-ray frequency.
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.
High-Harmonic Generation and x-rays
Amplified
femtosecond laser
pulse
x-rays
gas jet
An ultrashort-pulse x-ray beam can be generated by focusing a
femtosecond laser in a gas jet
Harmonic orders > 300, photon energy > 500 eV, observed to date
HHG is a highly nonlinear process
resulting from highly nonharmonic
motion of an electron in an intense field.
The strong field smashes the electron into the nucleus—a highly
non-harmonic motion!
Ion
x-ray
electron
Wavelengths of a few Angstroms has been created this way.
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.
ee+
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