1. The Electromagnetic Spectrum and Blackbody Radiation
Sources of light: gases, liquids, and solids
The electromagnetic spectrum
Long-wavelength sources
and applications
Visible light and the eye
Short-wavelength sources and
applications
Boltzmann's Law
Blackbody radiation

2. Where does light come from?
We’ve seen that Maxwell’s Equations (i.e., the wave equation) describe the propagation of light.
But where does light come from in the first place?
Some matter must emit the light. It does so through the matter’s “polarization”:
Note that matter’s polarization is analogous to the polarization of light. Indeed, it will cause the emission of light with the same polarization direction.
where N is the number density of charged particles, q is the charge of each particle, and is the position of the charge. Here, we’ve assumed that each charge is identical has identical motion.

3. Maxwell's Equations in a Medium
The induced polarization, , contains the effect of the medium and is included in Maxwell’s Equations:
This extra term also adds to the wave equation, which is known as the "Inhomogeneous Wave Equation:”
The polarization is the “source term” and tells us what light will be emitted.
Notice that the induced polarization, and hence , gets differentiated twice. But is just the charge acceleration!
So it’s accelerating charges that emit light!

4. 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

5. But the vast majority of light in the universe comes from molecular vibrations emitting light.
Electrons vibrate in their motion around nuclei
High frequency: ~ cycles per second.
Nuclei in molecules vibrate with respect to each other
Intermediate frequency: ~ cycles per second.
Nuclei in molecules rotate
Low frequency: ~ cycles per second.

6. Water’s vibrations
Movies from

7. Atomic and molecular vibrations correspond to excited energy levels in quantum mechanics.
Energy levels are everything in quantum mechanics.
Excited level
DE = hn
Energy
Ground level
The atom is vibrating at frequency, n.
The atom is in a “superposition” of the ground and excited state.
This is true for all types of vibrations.

8. Molecules (and everything else) have many energy levels and can emit light only by making a transition from one level to another.
A typical molecule’s energy levels:
E = Eelectonic + Evibrational + Erotational
2nd excited
electronic state
Lowest vibrational and rotational level of this electronic “manifold”
Energy
1st excited
electronic state
Excited vibrational and rotational level
Transition
There are many other complications, such as spin-orbit coupling, nuclear spin, etc., which split levels.
Ground
electronic state

9. Different atoms emit light at different widely separated frequencies.
Each colored “emission” line corresponds to a difference between two energy levels.
Frequency (energy)
Atoms have simpler energy level systems (and hence simpler spectra) because they have no nuclear vibrations (i.e., only electronic levels).

10. Collisions broaden the frequency range of light emission.
A collision abruptly changes the phase of the sine-wave light emission. So atomic emissions can have a broader spectrum.
Gases at atmospheric pressure have emission widths of ~ 1 GHz.
Solids and liquids emit much broader ranges of frequencies (~ 1013 Hz!).
Quantum-mechanically speaking, the levels shift during the collision.

11. 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 that, together, have transitions at every wavelength.

12. Absorption Spontaneous Emission Stimulated Emission
Before After
Absorption Spontaneous Emission Stimulated Emission

13. Boltzmann Population Factors
Ni is the number density of molecules in state i (i.e., the number of molecules per cm3).
T is the temperature, and kB is Boltzmann’s constant. E3 N3 E2 N2
Energy N1 E1
Population density

14. The Maxwell-Boltzman Distribution
In the absence of collisions,
molecules tend to remain
in the lowest energy state
available.
Collisions can knock a mole-
cule into a higher-energy state.
The higher the temperature,
the more this happens.
Low T
High T 3 3
Energy
Energy 2 2 1 1
Molecules
Molecules
In equilibrium, the ratio of the populations of two states is:
N2 / N1 = exp(–DE/kBT ), where DE = E2 – E1 = hn
As a result, higher-energy states are always less populated than the
ground state, and absorption is stronger than stimulated emission.

15. Einstein A and B coefficients
In 1916, Einstein considered the various transition rates between molecular states (say, 1 and 2) involving light of irradiance, I:
Absorption rate = B12 N1 I
Spontaneous emission rate = A N2
Stimulated emission rate = B21 N2 I
In equilibrium, the rate of upward transitions equals the rate of
downward transitions:
B12 N1 I = A N2 + B21 N2 I
Rearranging:
(B12 I ) / (A + B21 I ) = N2 / N1 = exp[–DE/kBT ]
Recalling the Maxwell-
Boltzmann Distribution

16. Einstein A and B coefficients and Blackbody Radiation
Now solve for the irradiance in: (B12 I ) / (A + B21 I ) = exp[-DE/kBT ]
Rearrange to: B12 I exp[DE/kBT] = A + B21 I
or: I = A / {B12 exp[DE/kBT] – B21}
or: I = [A/B21] / { [B12 /B21] exp[DE/kBT] – 1 }
Now, when T ® ¥, I should also. As T ® ¥, exp[DE/kBT ] ® 1.
So: B12 = B21 º B ¬ Coeff up = coeff down!
And: I = [A/B] / {exp[DE/kBT ] – 1}
Eliminating A/B: using DE = hn

17. Blackbody Emission Spectrum
The higher the temperature, the more the emission and the shorter the average wavelength.
"Blue hot" is hotter
than "red hot."

18. Wien's Law: Blackbody peak wavelength scales as 1/Temperature.
Writing the Blackbody spectrum vs. wavelength:

19. Color temperature
Blackbodies are so pervasive that a light spectrum is often characterized in terms of its temperature even if it’s not exactly a blackbody.
Image from the magazine Digital PhotoPro, “White Balance,”, May/June 2004

20. The Electromagnetic Spectrum
radio
gamma-ray
visible
microwave
infrared UV
X-ray
106
105
wavelength (nm)
The transition wavelengths are a bit arbitrary…

21. The Electromagnetic Spectrum
Now, we’ll run through the entire electromagnetic spectrum, starting at very low frequencies and ending with the highest-frequency gamma rays.

22. 60-Hz radiation from power lines
Yes, this very-low-frequency current emits 60-Hz electromagnetic waves.
No, it is not harmful. A flawed epide- miological 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 < Gauss; the earth’s magnetic field is ~ 0.4 G.

23. The Long-Wavelength Electro-magnetic Spectrum
Arecibo image used with permission from Gabriela González, Department of Physics and Astronomy, Louisiana State University
Arecibo radio telescope

24. Radio & microwave regions (3 kHz – 300 GHz)

25. 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 GHz (1.228 GHz is a reference to compensate for atmos- pheric water effects)
4 signals are required; one for time, three for position.
2-m accuracy (100 m for us).

26. Microwave ovens
Microwave ovens operate at 2.45 GHz, where water absorbs very well.
Percy LeBaron Spencer, Inventor of the microwave oven
Picture of Spencer from

27. 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)

28. Cosmic Microwave Background
Microwave background vs. angle. Note the variations.
Peak frequency is ~ 150 GHz
The 3° cosmic microwave
background is blackbody
radiation left over from
the Big Bang!
Interestingly, blackbody radiation retains a blackbody spectrum despite the expansion the universe. It does get colder, however.
Wavenumber (cm-1)

29. TeraHertz light (a region of microwaves)
TeraHertz light is light with a frequency of ~1 THz, that is, with a wavelength of ~300 mm.
THz light is heavily absorbed by water, but clothes are transparent in this wavelength range.
CENSORED
Fortunately, I couldn’t get permission to show you the movies I have of people with THz-invisible invisible clothes.

30. IR is useful for measuring the temperature of objects.
Hotter and hence brighter in the IR
Old Faithful
Images borrowed from Linda Hermans-Killam,
Such studies help to confirm that Old Faithful is in fact faithful and whether human existence is interfering with it.

31. IR Lie-detection
I don’t really buy this, but I thought you’d enjoy it…
He’s really sweating now…

32. The military uses IR to see objects it considers relevant.
IR light penetrates fog and smoke better than visible light.

33. Jet engines emit infrared light from 3 to 5.5 µm
This light is easily distinguished from the ambient infrared, which peaks
near 10mm and is relatively weak in this range

34. The Infrared Space Observatory
Stars that are just forming emit light mainly in the IR.

35. Using mid-IR laser light to shoot down missiles
Wavelength = 3.6 to 4.2 mm
Thanks to Michael Gura (Optics I 2003) for this reference!
Tactical High Energy Laser (THEL) Program   The fixed-site version Advanced Concept Technology Demonstration (ACTD) Tactical High Energy Laser (THEL) THEL, was developed by TRW Inc. under a $89 million contract. During several tests in the USA, the system has shot down 25 Katyusha rockets, but has not been deployed. The system has not progressed much since the end of the demonstration program, since the lack of mobility and the fixed base limitations of the system made in insufficient to counter long range rockets currently employed by Hezbulla at the Israeli northern border with Lebanon. While Katyusha rockets had a range of 20 kilometers, and could hit only a few urban targets, the long range rockets have a range of 70 kilometers and can hit strategic facilities and large urban areas in the Haifa bay. A laser-based defense against such weapons must rely on more systems, which could be rapidly mobilized to protect a much larger area. Similar threats could face US contingencies in other parts of the world. This requirement is driving the need for an air-mobile version of the beam weapon. A study completed in 2001 concluded that the rocket interceptor has "lots of promise" and further development should be pursued, primarily in enabling system's mobility. Mobility considerations for the future mobile systems include system mobility (road and off road capabilities) and air transportability, including the type of transport aircraft it should fit on (C-130, C-17 or C-5). Conclusions of these studies will define the necessary size- reduction technologies required for the future version. Further studies of the system include the use of such laser beam weapons to provide "hard kill" defenses against artillery projectiles, UAVs and cruise missiles. The Tactical High Energy Laser uses a high-energy, deuterium fluoride chemical laser to protect against attack by short range unguided (ballistic flying) rockets. In a typical engagement scenario, a rocket is launched toward the defended area. Upon detection by the THEL fire control radar (image on right), the radar establishes trajectory information about the incoming rocket, then "hands off" the target to the pointer-tracker subsystem, which includes the beam director (top of page above). The PTS tracks the target optically, then begins a "fine tracking" process for THEL's beam director, which then places THEL's high-energy laser on target. The energy of the laser causes intense heating of the target, which causes its warhead to explode. The debris from the target falls quickly to the ground, far short of the defended area.
Above: Sequence of a rocket intercept demonstration by e THEL laser, September In these photos, THEL/ACTD laser spot focus on the warhead (top) of the 5 inch diameter rocket, and detonate it (center), thus effectively "neutralizing" the rocket. The gases emitted by the explosion create excessive drag which tears the fragmentation casing into several parts which continue on their ballistic trajectory. (bottom of image series) Below: THEL Radar and fire control system
The Tactical High Energy Laser uses a high-energy, deuterium fluoride chemical laser to shoot down short range unguided (ballistic flying) rockets.

36. Laser welding
Near-IR wavelengths are commonly used.

37. Atmospheric Penetration depth (from space) vs. Wavelength

38. Visible Light Wavelengths and frequencies of visible light
Hecht and

39. Dye lasers cover the entire visible spectrum.

40. Fluorescent lights
“Incandescent” lights (normal light bulbs) lack the emission lines.

41. 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.
Images and text courtesy of the NDT Resource Center

42. 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.
Intermediate colors, such as yellow and orange, are perceived by comparing relative responses of two or more different receptors.

43. 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!

44. The Ultraviolet
The UV is usually broken up into three regions, UVA ( nm), UVB ( nm), and UVC ( nm).
UVC is almost completely absorbed by the atmosphere.
You can get sun burned by all three.

45. UV from the sun
The ozone layer absorbs wavelengths less than 320 nm (UVB and UVC), and clouds scatter what isn’t absorbed.
But much UV (mostly UVA, but some UVB) penetrates the atmosphere anyway.

46. IR, Visible, and UV Light and Humans
(Sunburn)
Tanning salons use UVA, but it can still cause a sunburn.

47. The very short-wavelength regions
Vacuum-ultraviolet (VUV)
180 nm > l > 50 nm
Absorbed by <<1 mm of air
Ionizing to many materials
Soft x-rays
5 nm > l > 0.5 nm
Strongly interacts with core
electrons in materials
Extreme-ultraviolet (XUV or EUV)
50 nm > l > 5 nm
Ionizing radiation to all materials

48. 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

49. 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.

50. The sun also emits x-rays.
The sun seen in the x-ray region.

51. Matter falling into a black hole emits x-rays.
Nearby star
Black hole
A black hole accelerates particles to very high speeds.

52. 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 x-rays (blue), visible light (green) and radio (red).

53. Some x-rays are created in auroras.

54. Atomic structure and x-rays
The Essential Physics of Medical Imaging 2nd Edition, pg. 21 by Jerrold T. Bushberg, J. Anthony Seibert, Edwin M. Leidholdt, Jr, and John M. Boone 2002 by Lippincott Williams & Wilkins in PA ISBN:
Ionization energy ~ 100 – 1000 e.v.
Ionization energy ~ .01 – 1 e.v.

55. 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 x-rays.
The faster the electrons, the higher the x-ray frequency.
Images from Medical Imaging Physics, Fourth Edition by William R. Hende and E. Russell Ritenour 2002 by Wiley-Liss, New York ISBN: Pages 71, 75

56. X-rays penetrate tissue and do not scatter much.
Roentgen’s x-ray image of his wife’s hand (and wedding ring)

57. X-rays for photo-lithography
You can only focus light to a spot size of the light wavelength. So x-rays are necessary for integrated-circuit 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.

58. 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

59. 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
electron
x-ray
How do we know this? Circularly polarized light (or even slightly elliptically polarized light) yields no harmonics!

60. Gamma rays result from matter-antimatter annihilation.
An electron and positron self-annihilate, creating two gamma rays whose energy is equal to the electron mass energy, mec2. e- e+
hn = 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.

61. 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.
No one knows what they are.
The gamma-ray sky
Image borrowed from
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.

62. The universe in different spectral regions…
Gamma Ray
X-Ray
Gamma-ray, visible, and microwave images used with permission from Gabriela González, Department of Physics and Astronomy, Louisiana State University
Gamma-ray view: includes all wavelengths less than about 1.2E-5nm (photon energies greater than 1E8eV)
X-ray and IR images from Optics I student Yujiro Ito, 2004.
X-ray view: from ROSAT shows wavelengths of 0.8nm (blue), 1.7nm (green), and 5.0nm (red)
Infrared view: from IRAS shows emission at 0.1nm, 0.06nm, and 0.012nm
Visible

63. The universe in more spectral regions…IR
Images used with permission from Gabriela González, Department of Physics and Astronomy, Louisiana State University
Microwave