1. Lecture 3 Quantum Physics- Underlying Theory for Remote Sensing
Professor Menglin S. Jin
Department of Meteorology
San Jose State University

2. diagram for remote sensing–solar radiation

3. Print this out for the students (teacher’s note)

4. wave ν ~ 1/λ c (speed of light) = λν
[SI stands for System International]
since light speed is constant, as λ increases (decreases, ν must decrease (increase)
the distance between two adjacent peaks on a wave is its wavelength λ
The total number of peaks (top of the individual up-down curve)
that pass by a reference lookpoint in a second is that wave's frequency ν
(in units of cycles per second, whose SI version is Hertz [1 Hertz = 1/s-1])

5. UNDERLYING REMOTE SENSING
THE QUANTUM PHYSICS
UNDERLYING REMOTE SENSING
Quanta, or photons (the energy packets first identified
by Einstein in 1905), are particles of pure energy
having zero mass at rest
the demonstration by Max Planck in 1901, and more
specifically by Einstein in 1905, that electromagnetic
waves consist of individual packets of energy was
in essence a revival of Isaac Newton's
(in the 17th Century) proposed but then
discarded corpuscular theory of light

6. THE QUANTUM PHYSICS UNDERLYING REMOTE SENSING
light, and all other forms of EMR, behaves both as waves and as particles. This is the famous "wave-particle" duality enunciated by de Broglie, Heisenberg, Born, Schroedinger, and others mainly in the 1920s
until the opening of the 20th Century, the question of whether radiation was merely a stream of particles or was dominantly wave motion was much debated

7. THE QUANTUM PHYSICS UNDERLYING REMOTE SENSING
How is EMR produced?
Essentially, EMR is generated when an electric charge is accelerated, or more generally, whenever the size and/or direction of the electric (E) or magnetic (H) field is varied with time at its source
In stars, high temperatures can bring about this electron transition, generating not only gamma rays and X-rays but radiation at longer wavelengths. Radiation of successively lower energy involves other atomic motions as follows: UV, Visible: transitions of outer electrons to a higher metastable energy level; Infrared: inter- or intra-molecular vibrations and rotations; Microwave: molecular rotations and field fluctuations.

8. PHOTON The photon is the physical form of a quantum,
the basic particle of energy studied in quantum mechanics
(which deals with the physics of the very small, that is,
particles and their behavior at atomic and subatomic levels).
The photon is also described as the messenger particle for EM force
or as the smallest bundle of light.
This subatomic massless particle, which also does not carry
an electric charge, comprises radiation emitted by matter
when it is excited thermally, or by nuclear processes (fusion, fission),
or by bombardment with other radiation (as well as by particle collisions).
It also can become involved as reflected or absorbed radiation.
Photons move at the speed of light: 299, km/sec
(commonly rounded off to 300,000 km/sec or ~186,000 miles/sec).
discovery by Einstein as he first described them in a famous 1905 paper
Consult for more details

9. Photon
Photon particles also move as waves and hence, have a "dual" nature. These waves follow a pattern that can be described in terms of a sine (trigonometric) function, as shown in two dimensions in the figure below.
In the physics of wave movement, sinusoidal (also called periodic) types are known as transverse waves because particles within a physical medium are set into vibrational motion normal (at right angles) to the direction of propagation. EMR can also move through empty space (a vacuum) lacking particulates in the carrier medium, so that the EMR photons are the only physical entities.

10. photon travels as an EM wave
having two components, oscillating as sine waves mutually at right angles, one consisting of the varying electric field, the other the varying magnetic field
Each photon is surrounded by an electric field (E) and a magnetic field (H) expressed as vectors oriented at right angles to each other. Both have the same amplitudes (strengths) which reach their maxima-minima at the same time. Unlike other wave types which require a carrier (e.g., water waves), photon waves can transmit through a vacuum (such as in space). When photons pass from one medium to another, e.g., air to glass, their wave pathways are bent (follow new directions) and thus experience refraction.

11. Wave
The wave amplitudes of the two fields are also coincident in time and are a measure of radiation intensity (brightness)

12. Planck's general equation
E=hv
The amount of energy characterizing a photon is determined using Planck's general equation
h is Planck's constant ( x Joules-sec), v (read as nu), representing frequency
A photon is said to be quantized, any given one possesses a certain quantity of energy
Some other photon can have a different energy value
Photons as quanta thus show a wide range of discrete energies.

13. Planck's general equation
Photons traveling at higher frequencies are therefore more energetic.
If a material under excitation experiences a change in energy level from a higher level E2 to a lower level E1, we restate the above formula as:
where v has some discrete value determined by (v2 - v1)

14. Planck Equation Wavelength is the inverse of frequency
C= λv
V= c/λ
c is the constant that expresses the speed of light
we can also write the Planck equation as

15. Class wake-up activity
Calculate the wavelength of a quantum of radiation whose photon energy is 2.10 x Joules; use 3 x 108 m/sec as the speed of light c
A radio station broadcasts at 120 MHz (megahertz or a million cycles/sec); what is the corresponding wavelength in meters (hint: convert MHz to units of Hertz)
Here the operative equation is: Wavelength = c/frequency. Thus, Wavelength = (3.00 x 108) divided by (120 x 106) = 2.5 meters.
Here the operative equation is: Wavelength = c/frequency. Thus, Wavelength = (3.00 x 108) divided by (120 x 106) = 2.5 meters

16. polychromatic vs. monochromatic
A beam of radiation (such as from the Sun) is usually polychromatic (has photons of different energies)
if only photons of one wavelength are involved the beam is monochromatic.
the distribution of all photon energies over the range of observed frequencies is embodied in the term spectrum
A photon with some specific energy level occupies a position somewhere within this range, i.e., lies at some specific point in the spectrum
polychromatic, meaning that they consist of numerous sinusoidal components waves of different frequencies
When the wavelengths being considered are confined to the visual range of human eyes (0.4 to 0.7 µm), the term "luminous"

17. photoelectric effect –measure photon energy level
the discovery by Albert Einstein in 1905
His experiments also revealed that regardless
of the radiation intensity, photoelectrons are
emitted only after a threshold frequency is exceeded
Einstein found that when light strikes a metal plate C, photoelectrons (negative charges) are ejected from its surface. In the vacuum those electrons will flow to a positively charged wire (unlike charges attract) that acts as a cathode. Their accumulation there produces an electric current which can be measured by an ammeter or voltmeter. The photoelectrons have kinetic energy whose maximum is determined by making the wire potential ever more negative (less positive) until at some value the current ceases. The magnitude of the current depends on the radiation frequencies involved and on the intensity of each frequency. His experiments also revealed that regardless of the radiation intensity, photoelectrons are emitted only after a threshold frequency is exceeded
for those higher than the threshold value (exceeding
the work function) the numbers of photoelectrons
released re proportional to the number
of incident photons

18. For more, read the Chapter on The Nature of Electromagnetic Radiation in the Manual of Remote Sensing, 2nd Ed

19. How are these physics related to
remote sensing?

20. Optical phenomena + light atmospheric constituent optical phenomena
process
atmospheric
structure

21. Atmospheric Structure
temperature gradient
humidity gradient
clouds
layers of stuff - pollutants, clouds

22. Optical phenomena + light atmospheric constituent optical phenomena
process
atmospheric
structure
From optical phenomena, remote sensing can retrieve atmosphere composition

23. Electromagnetic Spectrum: Transmittance, Absorptance, and Reflectance
Any beam of photons from some source passing through medium 1 (usually air) that impinges upon an object or target (medium 2) will experience one or more reactions that are summarized below:

24. Electromagnetic Spectrum: Transmittance, Absorptance, and Reflectance
(1) Transmittance (τ) - some fraction (up to 100%) of the radiation penetrates into certain surface materials such as water and if the material is transparent and thin in one dimension, normally passes through, generally with some diminution.
(2) Absorptance (α) - some radiation is absorbed through electron or molecular reactions within the medium ; a portion of this energy is then re-emitted, usually at longer wavelengths, and some of it remains and heats the target;
(3) Reflectance (ρ) - some radiation (commonly 100%) reflects (moves away from the target) at specific angles and/or scatters away from the target at various angles, depending on the surface roughness and the angle of incidence of the rays.
A fourth situation, when the emitted radiation results from internal atomic/molecular excitation, usually related to the heat state of a body, is a thermal process. The theory underlying thermal remote sensing is treated Surface Temperature Lecture.
The Law of Conservation of Energy: τ + α + ρ = 1.

25. Absorption is process of removal of energy (photons) from the beam by conversion of the EM energy to another form (usually thermal)

26. When a remote sensing instrument
Most remote sensing systems
are designed to collect reflected radiation.
When a remote sensing instrument
has a line-of-sight with an object that is reflecting
solar energy, then the instrument collects
that reflected energy and records the observation.

27. W.R.T Remote Sensing
What really measured by remote sensing detectors are radiances at different wavelengths leaving extended areas

28. Radiative Transfer
What happens to radiation (energy) as it travels from the “target” (e.g., ground, cloud...) to the satellite’s sensor?

29. Processes: transmission reflection scattering absorption refraction
dispersion
diffraction

30. transmission the passage of electromagnetic radiation through a medium
transmission is a part of every optical phenomena (otherwise, the phenomena would never have occurred in the first place!)

31. reflection
the process whereby a surface of discontinuity turns back a portion of the incident radiation into the medium through which the radiation approached; the reflected radiation is at the same angle as the incident radiation.

32. Reflection from smooth surface
light ray
angle of
reflection
angle of
incidence

33. Scattering
The process by which small particles suspended in a medium of a different index of refraction diffuse a portion of the incident radiation in all directions. No energy transformation results, only a change in the spatial distribution of the radiation.

34. Molecular scattering (or other particles)

35. Rayleigh Scattering vs Mie Scattering
Rayleigh scattering (named after the British physicist Lord Rayleigh) is the elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light, which may be individual atoms or molecules .
the Rayleigh scattering intensity for a single particle is 1/λ4
Scattering by particles similar to or larger than the wavelength of light is typically treated by the Mie scattering

36. Scattering from irregular surface

37. Absorption (attenuation)
The process in which incident radiant energy is retained by a substance.
A further process always results from absorption:
The irreversible conversion of the absorbed radiation goes into some other form of energy (usually heat) within the absorbing medium.

38. incident radiation substance (air, water, ice, smog, etc.) absorption
transmitted
radiation
absorption

39. window
Atmosphere
Window

40. Designation
Abbreviation
Wavelength
Near Infrared
NIR
0.78–3 µm
Mid Infrared
MIR
3–50 µm
Far Infrared
FIR
50–1000 µm OR
Near-infrared (NIR, IR-A)  µm
Short-wavelength infrared (SWIR, IR-B)  µm
Mid-wavelength infrared (MWIR, IR-C) 3-8 µm
Long-wavelength infrared (LWIR, IR-C) 8–15 µm
Far infrared (FIR) ,000 µm
Definition is not the same for different books!

41. Refraction
The process in which the direction of energy propagation is changed as a result of:
A change in density within the propagation medium, or
As energy passes through the interface representing a density discontinuity between two media.

42. Refraction in two different media
less dense
medium
more dense
medium

43. Refraction in two different mediaDt
less dense
medium
more dense
medium Dt

44. Gradually changing medium
low density
ray
wave
fronts
high density

45. Dispersion
the process in which radiation is separated into its component wavelengths (colors).

46. The “classic” example
white light
prism

47. Diffraction
The process by which the direction of radiation is changed so that it spreads into the geometric shadow region of an opaque or refractive object that lies in a radiation field.

48. light
shadow
region
Solid object

49. Atmospheric Constituents:
empty space
molecules
dust and pollutants
salt particles
volcanic materials
cloud droplets
rain drops
ice crystals

50. White clouds scattering off cloud droplets ~ 20 microns
Question: is this Mie scattering or Rayleigh scattering? Why?

51. Dark clouds
scattering and attenuation from larger cloud droplets and raindrops

53. Blue skies scattering from O2 and N2 molecules, dust
violet light is scattered 16 times more than red (why?)

54. Molecular scattering (nitrogen and oxygen)
[blue scatters more than red]

55. Hazy (milky white) sky Scattering from tiny particles
terpenes (hydrocarbons) and ozone

56. Orange sun (as at sunset or sunrise)
Scattering from molecules
This is the normal sunset we see frequently

57. Red sun (as at sunset or sunrise)
Scattering from molecules, dust, salt particles, volcanic material
At 4° elevation angle, sun light passes through 12 times as much atmosphere as when directly overhead

59. The change of sky colour at sunset (red nearest the sun, blue furthest away) is caused by Rayleigh scattering by atmospheric gas particles which are much smaller than the wavelengths of visible light. The grey/white colour of the clouds is caused by Mie scattering by water droplets which are of a comparable size to the wavelengths of visible light.