Lecture 2: Properties of Radiation
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Transcript of Lecture 2: Properties of Radiation
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Lecture 2: Properties of Radiation
Chapter 2 & 3 Petty
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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
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Properties of Radiation• The Nature of Electromagnetic Radiation- Electric and Magnetic fields (detectable at some distance
from their source)
F1=F2=Kc*q1q2
r2
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Properties of Radiation• The Nature of Electromagnetic Radiation- A changing magnetic field produces an electric field (thisis the phenomenon of electromagnetic induction, thebasis of operation for electrical generators, inductionmotors, and transformers).- Similarly, a changing electric field generates a magneticfield.- Because of this interdependence of the electric andmagnetic fields, it makes sense to consider them as asingle coherent entity—the electromagnetic field
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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)
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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.
Properties of Radiation
http://paws.kettering.edu/~drussell/Demos/superposition/superposition.html
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WAVE NATURE OF LIGHT
Light is an electromagnetic wave.
Different wavelengths in the visible spectrum are seen by the eye as different colors.
Wavelength
Red: = 700 nm
Blue: = 400 nm
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Properties of Radiation
• Electromagnetic Waves
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Basic concepts and definitionsElectromagnetic 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
k/1k
/c
m
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Frequency and wavelength
v = c
Frequency (Hz)
Wavelength
Speed of light
1 hertz (Hz) = one cycle per secondc = 3.0 x 108 ms-1 Weather Radar,
3GHzwavelength??
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Frequency
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Frequency Decomposition- What if electromagnetic disturbance is not a steady oscillating signal?
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Frequency Decomposition
• Eq. (2.2): any EM fluctuation can be thought of as a composite of a number of different “pure” periodic fluctuation
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Broadband vs. Monochromatic
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Broadband vs. Monochromatic
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Radio
Long WavelengthShort Wavelength
Gamma Ray X-ray Ultraviolet Infrared Microwaves
Visible
ELECTROMAGNETIC SPECTRUM
Lasers operate in the ultraviolet, visible, and infrared.
Radio
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RedBlue YellowGreen
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Major Spectral Bands --Visible Band
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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
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Answer:
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Radiation properties
• Quantum description• Wave description
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Quantum Properties of Radiation
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STIMULATED EMISSION
Incident Photon
Excited Atom
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Stimulated Photon same wavelength same direction in phase
Incident Photon
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• 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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Flux and Intensity
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Flux and Intensity
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Flux
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Intensity
• - Spherical Polar Coordinate
Fig. 2.3
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• 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?
Solid angle
2/ r
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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 angleunder which the earth is seen by the satellite sensor is
ea
)]/()2(1[2 2/12 hahha ee
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Solid angle and definition of steradian
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Solid angle and definition of steradian
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Chapter 3 Electromagnetic Spectrum
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Blackbody radiation
• Examine relationships between temperature, wavelength and energy emitted
• Blackbody: A “perfect” emitter and absorber of radiation... does not exist
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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
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Radiance
Toward satellite
Solid angle, measured in steradians(1 sphere = 4 sr = 12.57 sr)
Normalto surface
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Stefan-Boltzmann Law
M BB = T 4
Total irradianceemitted 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
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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:
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Planck’s function
B (T) = c1-5
exp (c2 / T ) -1
Irridance:Blackbody radiative fluxfor a single wavelength at temperature T (W m-2 m-1)
Second radiation constantAbsolute temperature
First radiation constant Wavelength of radiation
Total amount of radiation emitted by a blackbody is a function of its temperaturec1 = 1.19x10-16 W m-2 sr-1
c2 = 1.44x10-2 m K
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Planck curve
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Wein’s Displacement Law
mT = 2897.9 m K
Gives the wavelength of the maximum emission of a blackbody, which is inversely proportional to its temperature
Earth @ 300K: ~10 mSun @ 6000K: ~0.5 m
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Intensity and Wavelength of Emitted Radiation : Earth and Sun
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Solar Spectrum
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Atmosphere Window
window
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Rayleigh-Jeans Approximation
B (T) = (c1 / c2) -4 T
When is this valid: 1. For temperatures encountered on Earth 2. For millimeter and centimeter wavelengthsAt microwave wavelengths, the amount of radiation emitted is directly proportional to T... not T4
(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.
B (T)TB =
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Emissivity and Kirchoff’s Law
Actual irradiance bya non-blackbodyat 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)
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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
• http://www.space.com/scienceastronomy/071217-solar-cycle-24.html
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CLOUD RADIATIVE FORCINGClouds can either warm or cool the climate
depending on the cloud typeCooling• By reflecting solar radiation back to
space• Particularly low clouds• Global average short wave cooling is - 48
W m-2Warming• 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-2Net Effect• Global cooling of about - 20 W m-2• But what will the cloud feedback be with
global