Blackbody radiation
derivation of
Planck‘s radiation low
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Classical theories of Lorentz and Debye:
•
Lorentz (oscillator model):
– Electrons and ions of matter were treated as a simple harmonic oscillators
(springs) subject to the driving force of applied E-M fields; matter becomes
polarized by induction of electric dipoles
• It models optical properties of materials and provides theory of refraction, reflectance
and absorption
• Analogy between the classical and the quantum-mechanical descriptions:
– E.g., excitation frequency vs. resonance frequency, probabilities of transition to all other
quantum states vs. damping factors
•
Debye(relaxation model):
– The E-M field causes polarization of matter containing permanent electric dipoles
leading to the partial alignment of the dipoles along the electric field against the
counteracting tendency toward disorientation caused by thermal buffeting. The
restoring force tries to return a polarized region to an unpolarized state is thus
the statistical tendency toward random orientation of the dipoles; the dipole
restoring tendency leads to oscillation of the electric polarization
•
It models the optical constants of liquids at certain frequencies
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Laws of radiation
• Kirchoffs law of thermal radiation (1860)
At thermal equilibrium, the emissivity of a body
(or surface) equals its absorptivity.
Introduction of black body radiation concept;
A black body is an object that absorbs all light that
falls on it ( no light is reflected or transmitted). The
object appears black when it is relatively cold.
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Wien's law
-
It accurately describes the spectrum of thermal radiation from objects at the
short wavelength
-the hotter an object is, the shorter the wavelength at which It
will emit most of its radiation,
-the frequency for maximal intensity or peak power
radiation
λ
T
B
is the peak wavelength in meters
is the temperature of the blackbody in Kelvins (K)
is a constant of proportionality, called Wien's displacement
constant and equals 2.897 768 5(51) × 10–3 m K
Rayleigh-Jeans, Wien and
Planck law‘s for a body of 8 mK
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•
Rayleigh–Jeans Law
it describes the spectral radiance of
electromagnetic radiation at all wavelengths
from a black body at a given temperature:
c is the speed of light,
k is Boltzmann's constant
T is the temperature in kelvins.
It predicts an energy output that diverges towards
infinity as wavelengths grow smaller. This was not
supported by experiments and the failure has
become known as the ultraviolet catastrophe.
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As the temperature decreases, the peak of the black-body radiation curve
moves to lower intensities and longer wavelengths.
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Derivation of Planck‘s radiation law
M. Planck, Ann. Phys.
Vol. 4, p.553 (1901)
•
Assumptions:
•
Blackbody cavity: schematic
A cavity in a material that is
maintained at constant
temperature T
The emission of radiation from the
cavity walls is in equilibrium with
the radiation that is absorbed by
the walls
The radiation field in an empty
volume in thermal equilibrium with
a container at T can be viewed as
a superposition of standing
harmonic waves
T, ϖi
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Practical cavity: example
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•
The radiation field in an empty
volume (V=L3) is in thermal
equilibrium with container at
temperature T.
This can be viewed as a superposition of
standing harmonic waves (oscillators
modes).
Radiation field in an empty volume
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The mode density
The waves are solutions
of the wave equation:
*
Taking into account
the boundary conditions:
The solutions of equation have the form:
**
with sin (kiL) = 0 from which follows kiL = ni for i = 1; 2; 3 and ni = 1; 2; …
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Inserting equations ** in * one obtains:
From which follows:
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The number of modes with angular frequency between 0 and ω is
where
represents the number density of oscillators which
corresponds to all combinations of n1,n2 and n3 which fulfill the equation:
***
Equation*** is the equation of a sphere (see Fig. 2) with radius:
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n1,n2,n3 are positive and therefore
****
Since ω = 2πν, one obtains from Equation ****
Diagram of the possible
(n1, n2, n3) combinations
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The mode density for a given polarization is therefore
Since two polarization directions have to be considered for each mode, the
mode density is twice larger than given above and amounts to:
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The energy density
Each mode has an energy kT and the energy of the radiation field in a volume
V at temperature T and between frequency ν and ν+νdν is:
The energy density (i.e., the energy per unit volume) is therefore given by
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The energy density (i.e., the energy per unit volume) is therefore given by
a relation known as Rayleigh-Jeans' law. The expression is valid for
but incorrect for
, as it predicts that in this case the energy density
should become infinite. This physically incorrect property of the equation would
lead to what has been termed \UV catastrophy".
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Planck's radiation law is derived by assuming that each radiation mode can be
described by a quantized harmonic oscillator with energy
Referencing the energy of each oscillator to the ground state (v = 0) of the oscillators:
one can determine the average energy of an oscillator using statistical mechanics:
with vhν the energy of the oscillator and
the oscillator has the energy vhν (Boltzmann factor)
the probability ( >=1) that)
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Making the substitution
The energy density is the product of the mode density per unit volume and the
average energy of the modes and is therefore given by:
Planck's law for the energy density of the radiation eld (M.
Planck, Verh. Deutsch. Phys. Ges. 2, 202 and 237 (1900)
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The total energy UT V per unit volume is
The heat capacity of free space can be determined to be:
At high T, UTV becomes very large and eventually suficiently large that
electron-positron pairs can be formed, at which point vacuum fills with matter.
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Applications
• Temperature measurements of astrophysical objects
• Cosmic microwave background radiation
• Color temperatureis a characteristic of visible light that has important applications in photography,
videography, publishing and other fields. The color temperature of a light source is determined by comparing its
chromaticity with a theoretical, heated black-body radiator. The temperature (in kelvin) at which the heated black-body
radiator matches the color of the light source is that source's color temperature
• Infrared thermometers measure temperature using blackbody radiation (generally infrared)
emitted from objects. They are sometimes called laser thermometers if a laser is used to help aim the thermometer, or
non-contact thermometers to describe the device’s ability to measure temperature from a distance. By knowing the
amount of infrared energy emitted by the object and its emissivity, the object's temperature can be determined.
• Combustion: laser induced incandescence for soot particle measurements, two-color
pirometry
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