1. The Electromagnetic Spectrum and Blackbody Radiation
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
Short-wavelength sources and applications
Prof. Rick Trebino
Georgia Tech

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 and has identical motion.

3. Polarized and unpolarized media
Unpolarized medium
Polarized medium
On the right, the displacements of the charges are correlated, so it is polarized at any given time (and its polarization is oscillating).

4. 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!

5. 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

6. 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.

7. Water’s vibrations
Movies from

8. 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 at least partially in an excited state.

9. Excited atoms emit photons spontaneously.
When an atom in an excited state falls to a lower energy level, it emits a photon of light.
Excited level
Energy
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.

10. 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).

11. 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.

12. Molecules have many energy levels.
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
As a result, molecules generally have very complex spectra.

13. Atoms and molecules can also absorb photons, making a transition from a lower level to a more excited one.
Excited level
This is, of course, absorption.
Energy
Image from
Ground level
Absorption lines in an otherwise continuous light spectrum due to a cold atomic gas in front of a hot source.

14. Decay from an excited state can occur in many steps.
Infra-red
Energy
Ultraviolet
Visible
Microwave
The light that’s eventually re-emitted after absorption may occur at other colors.

15. carbon dioxide water vapor methane nitrous oxide
The Greenhouse Effect
Visible
Infra-red
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.
Greenhouse gases:
carbon dioxide water vapor methane nitrous oxide
Methane, emitted by microbes called methanogens, kept the early earth warm.
Picture from

16. Global warming

17. 2008 global temperatures compared with 1951-1980

18. Global warming facts
The concentration of CO2 in the atmosphere rose from 290 ppm in 1900 to 316 ppm in 1959 and to 387 ppm in 2009.
The ten warmest years ever have occurred in the period
About 75% of the annual increase in atmospheric carbon dioxide is due to the burning of fossil fuels. The remaining 25% is attributed to changes in land use, reducing the net uptake of CO2.

19. Historical temperature and CO2 levels
See also:

20. Einstein showed that stimulated emission can also occur.
Before After
Spontaneous emission
Absorption
Stimulated emission

21. In what energy levels do molecules reside? 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

22. 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.

23. 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 many different molecules that that, together, have transitions (absorptions) at every wavelength.

24. 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:
Spontaneous emission rate = A N2
Absorption rate = B12 N1 I
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
Solving for N2/N1:
Recalling the Maxwell-
Boltzmann Distribution
(B12 I ) / (A + B21 I ) = N2 / N1 = exp[–DE/kBT ]

25. Einstein A and B coefficients and Blackbody Radiation
Now solve for the irradiance in: (B12 I ) / (A + B21 I ) = exp[-DE/kBT ]
Multiply by A + B21 I : B12 I exp[DE/kBT] = A + B21 I
Solve for I: 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
(based on other using DE = hn
information):

26. Writing the blackbody spectrum vs. wavelength
Units of I(n): energy/volume/frequency
Change variables from n to l:
Units of I(l): energy/volume/wavelength

27. 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.

28. Wien's Law: Blackbody peak wavelength scales as 1/Temperature.
Writing the blackbody spectrum vs. wavelength:
dlambda/dnu = -c/nu^2 = -lambda^2/c

29. 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.
Keep in mind that blackbody spectra are broad, so they usually look white with a tint of the peak color (wavelength).
Image from the magazine Digital PhotoPro, “White Balance,”, May/June 2004

30. The electromagnetic spectrum
The transition wavelengths are a bit arbitrary. Visible light used to be 400 to 700 nm, but now it’s officially 380 to 780 nm.

31. 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.

32. 60-Hz radiation from power lines
Yes, this very-low-frequency current emits 60-Hz electromagnetic waves.
No, it is not harmful. A very 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 < Gauss; the earth’s magnetic field is ~ 0.4 G.

33. 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

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

35. 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).
Corrections must be made for General Relativity! Otherwise accuracy would be ~ 8 m.

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

37. 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)

38. 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)

39. 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 clothes.