The nature of electromagnetic radiation part1 Flashcards
Electromagnetic radiation in the atmosphere interacts with
gases, aerosol particles, and cloud particles.
Extinction and emission are two main types of
the interactions between an electromagnetic radiation field and a medium (e.g., the atmosphere).
Extinction is
a process that decreases the radiative intensity, while emission increases it.
extinction also means
attenuation
Radiation is ………………….. by …………………………………………………………………………..
emitted
all bodies that have a temperature above absolute zero (O K) (often referred to as thermal emission).
Extinction is due to
absorption and scattering.
Absorption is
a process that removes the radiative energy from an electromagnetic field and transfers it to other forms of energy.
Scattering is
a process that does not remove energy from the radiation field, but may redirect it.
Scattering can be thought of as ……………………….of …………………………………………….
absorption
radiative energy followed by re- emission back to the electromagnetic field with negligible conversion of energy
Scattering can be thought of as absorption of radiative energy followed by re- emission back to the electromagnetic field with negligible conversion of energy. Thus,
scattering can remove radiative energy of a light beam traveling in one direction, but can be a “source” of radiative energy for the light beams traveling in other directions.
what is the difference between elastic and inelastic scattering
Elastic scattering is the case when the scattered radiation has the same frequency as that of the incident field. Inelastic (Raman) scattering results in scattered light with a frequency different from that of the incident light.
Blackbody is
a body whose absorbs all radiation incident upon it.
Properties of blackbody radiation:
- Radiation emitted by a blackbody is isotropic, homogeneous and unpolarized;
- Blackbody radiation at a given wavelength depends only on the temperature;
- Any two blackbodies at the same temperature emit precisely the same radiation;
- A blackbody emits more radiation than any other type of an object at the same temperature;
The atmosphere is not strictly in the thermodynamic equilibrium because
its temperature and pressure are functions of position. Therefore, it is usually subdivided into small subsystems each of which is effectively isothermal and isobaric referred to as Local Thermodynamical Equilibrium (LTE).
A concept of LTE plays a fundamental role in
atmospheric studies: e.g., the main radiation laws discussed below, which are strictly speaking valid in thermodynamical equilibrium, can be applied to an atmospheric air parcel in LTE.
Planck function definition
Bλ(T), gives the intensity (or radiance) emitted by a blackbody having a given temperature.
Plank function can be expressed in
wavelength, frequency, or wavenumber domains
Plank function can be expressed in wavelength, frequency, or wavenumber domains as
what does each symbol mean
The relations between Bv~ (T ); Bv (T ) and Bλ(T ) are derived using that
Explain the graph
the graph shows plancks radiance “emissivity” and wavelength
when temperature of the object is low wavelength is larger and radiance is low
The Stefan-Boltzmann law states that
the radiative flux emitted by a blackbody, per unit surface area of the blackbody, varies as the fourth power of the temperature.
Stefan-Boltzmann law formula
explain the terms in Stefan-Boltzmann law
Wien’s displacement law
states that the wavelength at which the blackbody
emission spectrum is most intense varies inversely with the blackbody’s temperature.
The Wien’s displacement law states that the wavelength at which the blackbody
emission spectrum is most intense varies inversely with the blackbody’s temperature. The
constant of proportionality is
Wien’s constant(2897 K µm):
λm= 2897 / T
Explain the terms of weins displacement law
where λm is the wavelength (in micrometers, µm) at which the peak emission intensity
occurs, and T is the temperature of the blackbody (in degrees Kelvin, K)
Wein’s displacement law is simply derived from
dBλ/dλ= 0
Wien’s displacement law (easy to remember statement)
The hotter the object the shorter the wavelength of the maximum intensity emitted
Kirchhodd’s law
states that the emissivity ελ, of a medium is equal to the
absorptivity, Αλ, of this medium under thermodynamic equilibrium
ελ= Αλ
explain the terms of kirchhoff’s law
where ελis defined as the ratio of the emitting intensity to the Planck function;
Αλis defined as the ratio of the absorbed intensity to the Planck function.
kirchhoff’s law
For a blackbody:
For a gray body:
For a non-blackbody
For a blackbody: ελ= Αλ = 1
For a gray body: ε= Α < 1 (i.e., no dependency on the wavelength)
For a non-blackbody: ελ= Αλ < 1
Kirchhoff’s lawapplies to
gases, liquids and solids if they in TE or LTE
In remote sensing applications, one needs to distinguish between the
emissivity of the surface(e.g., various types of lands, ice, ocean etc.) and the
emissivity of an atmospheric volume (consisting of gases, aerosols, and/or clouds).
Brightness temperature
Tb,is defined as the temperature of a blackbody that emits the
same intensity as measured. Brightness temperature is found by inverting the Planck
function.
where Iλ is the
measured intensity
Brightness temperature
for a blackbody
brightness temperature = kinetic temperature ( Tb= T)
Brightness temperature
for natural materials:
Tb4 = εT4
(ε is the broadband emissivity)
The ocean and land surfaces can modify the atmospheric radiation field by
a) reflecting a portion of the incident radiation back into the atmosphere;
b) transmitting some incident radiation;
c) absorbing a portion of incident radiation ( Kirchhoff’s law);
d) emitting the thermal radiation (Kirchhoff’s law);
What does A represent
Incident Radiation
What does B represent
Reflected radiation
What does C represent
Emitted radiation
what does d represent
Transmitted radiation
what does e represent
Absorbed radiation
Conservation of energy requires that
monochromatic radiation incident upon any surface,
Ii, is either reflected, Ir, absorbed, Ia, or transmitted, It
Conservation of energy requires that monochromatic radiation incident upon any surface, Ii, is either reflected, Ir, absorbed, Ia, or transmitted, It, Thus
I<sub>i</sub>= I<sub>r</sub> + I<sub>a</sub>+ I<sub>t</sub> 1 = I<sub>r</sub> / I<sub>i</sub> + I<sub>a</sub>/ I<sub>i </sub>+ I<sub>t</sub> / I<sub>i</sub> = R + A+ T
What is T, A and R
where T is the transmission, A is the absorption, and R is the reflection of the surface.
In general, T, A, and R are functions of
wavelength:
Rλ + Aλ+ Tλ =1
Blackbody surfaces (______________) and surfaces in LTE (from Kirchhoff’s law):
no reflection
Blackbody surfaces (no reflection) and surfaces in LTE (from Kirchhoff’s law):
Aλ = ελ
Opaque surfaces (__________________-)
no transmission
Opaque surfaces (no transmission)
Rλ + Aλ = 1
Thus for the opaque surfaces
ελ = 1- Rλ
In general, emissivity depends on
the direction of emission, surface temperature,
wavelength and some physical properties of the surface
In the thermal IR (___________), nearly all surfaces are
4µm<λ< 100µm
efficient emitters with the emissivity > 0.8 and their emissivity does not depend on the direction.
In the thermal IR (4µm<λ< 100µm), nearly all surfaces are efficient emitters with
the emissivity > 0.8 and their emissivity does not depend on the direction.
Therefore,
the intensity emitted from a unit surface area at a given wavelength is
Iλ = ελ Bλ(Ts)
In the shortwave region (________________), emissivity is
0.1 µm <λ< 4 µm
negligibly small.
In microwave (_______________), emissivity
0.1 cm<λ< 100 cm
depends on the type and state of the surface.
IR window
10 to12 µm
Surface: water
Emissivity:
0.993-0.998
Surface: Ice
Emissivity:
0.98
Surface: green grass
Emissivity:
0.975 - 0.986
Surface: sand
emissivity:
0.949-0.962
Surface: snow
emissivity:
0.969-0.997
Surface: granite
Emissivity:
0.898
Many natural surfaces have __________emissivity in the IR window and hence __________________________________–
high
low (negligible) reflectivity in this spectral region.
