1. Lecture 2: Properties of Radiation
Chapter 2 & 3 Petty

2. Properties of Radiation
What is radiation?
How it behaves at the most fundamental physical level?
What conventions are used to classify it according to wavelength and other properties?
How do we define the characteristics (e.g. intensity) that appear in quantitative descriptions of radiation and its interaction with the atmosphere

3. Properties of Radiation
The Nature of Electromagnetic Radiation
Electric and Magnetic fields (detectable at some distance from their source)
F1=F2=Kc*q1q2 r2

4. Properties of Radiation
The Nature of Electromagnetic Radiation
- A changing magnetic field produces an electric field (this
is the phenomenon of electromagnetic induction, the
basis of operation for electrical generators, induction
motors, and transformers).
- Similarly, a changing electric field generates a magnetic
field.
- Because of this interdependence of the electric and
magnetic fields, it makes sense to consider them as a
single coherent entity—the electromagnetic field

5. Properties of Radiation
The electromagnetic spectrum is a continuum of all electromagnetic waves arranged according to frequency and wavelength. The sun, earth, and other bodies radiate electromagnetic energy of varying wavelengths.
Electromagnetic energy passes through space at the speed of light in the form of sinusoidal waves. The wavelength is the distance from wavecrest to wavecrest (see the figure below)

6. Properties of Radiation
Electromagnetic Waves
EM wave propagate as rays will spread the wave’s energy over a larger area
and weaken as it gets further away.
- EM waves follow principle of superposition.
- EM waves are “transverse” waves.
The principle of superposition may be applied to waves whenever two (or more) waves travelling through the same medium at the same time. The waves pass through each other without being disturbed. The net displacement of the medium at any point in space or time, is simply the sum of the individual wave displacements. This is true of waves which are finite in length (wave pulses) or which are continuous sine waves.
A transverse wave is a moving wave that consists of oscillations occurring perpendicular (or right angled) to the direction of energy transfer. If a transverse wave is moving in the positive x-direction, its oscillations are in up and down directions that lie in the y–z plane.

7. WAVE NATURE OF LIGHT Blue: l = 400 nm
Wavelength
Red: l = 700 nm
Blue: l = 400 nm
Light is an electromagnetic wave.
Different wavelengths in the visible spectrum are seen by the eye as different colors.
Light is an electromagnetic wave. It consists of oscillating electric and magnetic fields traveling through space. This figure is a representation of the electric field of a light wave.
The wavelength is the distance between two peaks on the wave. Different wavelengths are seen by the eye as different colors. Blue light has a wavelength of about 400 nm (0.4 mm). Red light has a wavelength of about 700 nm (0.7 mm). (1 nm = m; 1 mm = 10-6 m)
For about three hundred years there was a disagreement among scientists about the nature of light. Some believed light was a wave and others believed that light was a particle. Both factions were eventually able to support their positions with experimental evidence. It was not until the beginning of the twentieth century that the answer was discovered. When light travels through space, it acts like a wave. When light is emitted or absorbed by an atom, it acts like a particle.
The “particle” of light is called a photon. It is not a material particle but rather a “quantum” that acts as though it is located in one place and has definite energy and momentum, like an ordinary particle. A photon is more accurately described as a packet of energy. The energy of a photon is related to the wavelength of the light. Shorter wavelength photons have more energy than long wavelength photons.
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8. Properties of Radiation
Electromagnetic Waves

9. Basic concepts and definitions
Electromagnetic radiation:
* Energy propagated in the form of an advancing electric and
magnetic field disturbance.
* Travels in wave form at the speed of light (c).
* Wavelength ( ) is the physical distance between adjacent
maxima or minima in the electric or magnetic field. unit:
* Wave number ( ) is the number of wavelengths in a unit
distance, i.e., unit: 1/cm
* Frequency ( ) is the number of successive maxima or
minima passing a fixed point in a unit of time,
unit: cycle-per-second (cps) or 1/s

10. Frequency and wavelength
Speed of light c
Frequency (Hz)
v = 
Wavelength
1 hertz (Hz) = one cycle per second
c = 3.0 x 108 ms-1
Weather Radar,
3GHz
wavelength??

11. Frequency

12. Frequency Decomposition
- What if electromagnetic disturbance is not a steady oscillating signal?

13. Frequency Decomposition
Eq. (2.2): any EM fluctuation can be thought of as a composite of a number of different “pure” periodic fluctuation

14. Broadband vs. Monochromatic

15. Broadband vs. Monochromatic

16. ELECTROMAGNETIC SPECTRUM
Blue
Green
Yellow
Red
Visible
Radio
Gamma Ray
X-ray
Ultraviolet
Infrared
Microwaves
Radio
Short Wavelength
Long Wavelength
The electromagnetic spectrum extends from gamma rays at the short wavelength end to radio waves at the long wavelength end. The visible spectrum is a narrow slice somewhere in the middle, with blue light at the short wavelength end and red light at the long wavelength end.
The next shortest wavelength region from the visible is the ultraviolet. Ultraviolet light causes sunburn, skin cancer, and cataracts.
The next longest wavelength region from the visible is the infrared. Infrared light is invisible to the eye but can be felt as heat. It can cause burns to the skin or eyes.
Lasers operate in the ultraviolet, visible, and infrared regions of the spectrum. Lasers in each spectral region present unique safety issues.
Lasers operate in the ultraviolet, visible, and infrared.
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17. Major Spectral Bands --Visible Band

18. Relevant to remote sensing
As a proportion of total solar irradiance:
Total energy from 0 – 0.75μm                54%   – all energy up to infra-red
Total energy from 0.39μm – 0.75μm      43%   – visible light only
Total energy from 0 – 4μm                   99%   – all “shortwave”
Total energy from 4-infinity                       1%   – all “longwave”
Total energy from 13μm-infinity              0.03% – major 15μm CO2 band and above
Terminology:
>0.75μm is infra-red (slightly different conventions exist about the maximum value for visible light, but nothing substantial)
0-4μm is “shortwave” – a climate science convention referring to solar radiation
4μm-infinity is “longwave“- a climate science convention referring to terrestrial radiation

20. Answer:

21. Radiation properties
Quantum description
Wave description

22. Quantum Properties of Radiation

23. STIMULATED EMISSION Incident Photon Incident Photon Excited Atom
Stimulated Photon
same wavelength
same direction
in phase
When energy is absorbed by an atom, some of the electrons in that atom move into larger, higher energy orbits. When energy is released by the atom, the electrons move to smaller orbits. The lowest energy state is called the ground state. This is when all the electrons are as close to the nucleus as possible. Higher energy states are called excited states. Excited atomic states are not stable. Excited atoms tend to release energy in the form of photons and drop to lower energy states.
Ordinary light is produced by spontaneous emission as excited atoms drop to lower energy levels and release photons spontaneously. The result is light that is a mixture of many different wavelengths, is emitted in all directions, and has random phase relationships.
Laser light is produced by stimulated emission when excited atoms are struck by photons in the laser beam. This stimulates the excited atoms to emit their photons before they are emitted randomly by spontaneous emission. The result is that each stimulated photon is identical to the stimulating photon. This means that all photons produced by stimulated emission have the same wavelength, travel in the same direction, and are in phase. Thus the stimulated emission process leads to the unique properties of laser light.
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24. When energy is absorbed by an atom, some of the electrons in that atom move into larger, higher energy orbits. When energy is released by the atom, the electrons move to smaller orbits. The lowest energy state is called the ground state. This is when all the electrons are as close to the nucleus as possible. Higher energy states are called excited states. Excited atomic states are not stable. Excited atoms tend to release energy in the form of photons and drop to lower energy states.
Ordinary light is produced by spontaneous emission as excited atoms drop to lower energy levels and release photons spontaneously. The result is light that is a mixture of many different wavelengths, is emitted in all directions, and has random phase relationships.
Laser light is produced by stimulated emission when excited atoms are struck by photons in the laser beam. This stimulates the excited atoms to emit their photons before they are emitted randomly by spontaneous emission. The result is that each stimulated photon is identical to the stimulating photon. This means that all photons produced by stimulated emission have the same wavelength, travel in the same direction, and are in phase. Thus the stimulated emission process leads to the unique properties of laser light.

25. Flux and Intensity

26. Flux and Intensity

27. Flux

28. Intensity
- Spherical Polar Coordinate
Fig. 2.3

29. Solid angle
The ratio of the area of the sphere intercepted by the cone to the square of the radius
Units: Steradians (sr)
What is the area cut out of a sphere by one steradian?
What is the solid angle representing all directions at a point?

30. HW2: A meteorological satellite circles the earth at a height h above
the earth’s surface. Let the radius of the earth be
and show that the solid angle
under which the earth is seen by the satellite sensor is

31. Solid angle and definition of steradian

32. Solid angle and definition of steradian

33. Chapter 3 Electromagnetic Spectrum

34. Blackbody radiation
Examine relationships between temperature, wavelength and energy emitted
Blackbody: A “perfect” emitter and absorber of radiation... does not exist

35. Measuring energy
Radiant energy: Total energy emitted in all directions (J)
Radiant flux: Total energy radiated in all directions per unit time (W = J/s)
Irradiance (radiant flux density): Total energy radiated onto (or from) a unit area in a unit time (W m-2)
Radiance: Irradiance within a given angle of observation (W m-2 sr-1)
Spectral radiance: Radiance for range in 

36. Radiance Toward satellite Normal to surface
Solid angle, measured in steradians
(1 sphere = 4 sr = sr)

37. (sometimes indicated as E*)
Stefan-Boltzmann Law
M BB = T 4
Total irradiance
emitted by a blackbody
(sometimes indicated as E*)
Stefan-Boltzmann constant
The amount of radiation emitted by a blackbody is
proportional to the fourth power of its temperature
Sun is 16 times hotter than Earth but gives off 160,000 times
as much radiation

38. Planck’s Function
Blackbody doesn't emit equal amounts of radiation at all wavelengths
Most of the energy is radiated within a relatively narrow band of wavelengths.
The exact amount of energy emitted at a particular wavelength lambda is given by the Planck function:

39. Planck’s function c1-5 B  (T) = exp (c2 / T ) -1
First radiation constant
Wavelength of radiation
c1-5
B  (T) =
exp (c2 / T ) -1
Absolute temperature
Second radiation constant
Irridance:
Blackbody radiative flux
for a single wavelength at temperature T (W m-2 m-1)
Total amount of radiation emitted by a blackbody is a function of
its temperature
c1 = 1.19x10-16 W m-2 sr-1
c2 = 1.44x10-2 m K

40. Planck curve

41. Wein’s Displacement Law
mT = m K
Gives the wavelength of the maximum emission of a
blackbody, which is inversely proportional to its temperature
300K: ~10 m
6000K: ~0.5 m

42. Intensity and Wavelength of Emitted Radiation : Earth and Sun

43. Solar Spectrum

44. window
Atmosphere
Window

46. Rayleigh-Jeans Approximation
B (T) = (c1 / c2) -4 T
When is this valid:
1. For temperatures encountered on Earth
2. For millimeter and centimeter wavelengths
At microwave wavelengths, the amount of radiation emitted
is directly proportional to T... not T4
B (T)
TB =
(c1 / c2) -4
Brightness temperature (TB) is often used for microwave and
infrared satellite data, where it is called equivalent blackbody
temperature. The brightness temperature is equal to
the actual temperature times the emissivity.

47. Emissivity and Kirchoff’s Law

Actual irradiance by
a non-blackbody
at wavelength 
Emittance: Often referred to as emissivity
Emissivity is a function of the wavelength of radiation and the viewing angle and) is the ratio of energy radiated by the material to energy radiated by a black body at the same temperature
absorbed/ incident
Absorptivity (r , reflectivity; t , transmissivity)

48. Solar Constant
The intensity of radiation from the Sun received at the top of the atmosphere
Changes in solar constant may result in climatic variations

49. CLOUD RADIATIVE FORCING
Clouds can either warm or cool the climate depending on the cloud type
Cooling
By reflecting solar radiation back to space
Particularly low clouds
Global average short wave cooling is - 48 W m-2
Warming
By acting as a greenhouse absorber and emitter of
long wave radiation
Particularly thin cirrus clouds
Global average long wave warming is + 28 W m-2
Net Effect
Global cooling of about - 20 W m-2
But what will the cloud feedback be with global