Everything about Black Body totally explained
In
physics, a
black body is an
object that absorbs all
light that falls on it. No electromagnetic radiation passes through it and none is
reflected. Because no light is reflected or transmitted, the object appears black when it's cold.
If the black body is hot, these properties make it an ideal source of
thermal radiation. If a perfect black body at a certain temperature is surrounded by other objects in
thermal equilibrium at the same temperature, it'll on average emit exactly as much as it absorbs, at every wavelength. Since the absorption is easy to understand—every ray that hits the body is absorbed—the emission is just as easy to understand.
A black body at temperature
T emits exactly the same wavelengths and intensities which would be present in an environment at equilibrium at temperature
T, and which would be absorbed by the body. Since the radiation in such an environment has a spectrum that depends only on temperature, the temperature of the object is directly related to the wavelengths of the light that it emits. At room temperature, black bodies emit
infrared light, but as the temperature increases past a few hundred degrees
Celsius, black bodies start to emit at visible wavelengths, from red, through orange, yellow, and white before ending up at blue, beyond which the emission includes increasing amounts of
ultraviolet.
The term "black body" was introduced by
Gustav Kirchhoff in
1860. The light emitted by a black body is called
black-body radiation.
If a small window is opened into an oven, any light that enters the window has a very low probability of leaving without being absorbed. Conversely, the hole acts as a nearly ideal black-body radiator. This makes peepholes into furnaces good sources of blackbody radiation, and some people call it
cavity radiation for this reason.
Black-body emission gives insight into the thermal equilibrium state of a continuous field. In classical physics, each different
Fourier mode in thermal equilibrium should have the
same energy, leading to the
nonsense prediction that there would be an infinite amount of energy in any continuous field. Black bodies could test the properties of thermal equilibrium because they emit radiation which is distributed thermally. Studying the laws of the black body historically led to
quantum mechanics.
Explanation
In the laboratory, black-body radiation is approximated by the radiation from a small hole entrance to a large cavity, a
hohlraum. Any light entering the hole would have to reflect off the walls of the cavity multiple times before it escaped, in which process it's nearly certain to be absorbed. This occurs regardless of the
wavelength of the radiation entering (as long as it's small compared to the hole). The hole, then, is a close approximation of a theoretical black body and, if the cavity is heated, the
spectrum of the hole's radiation (for example, the amount of light emitted from the hole at each
wavelength) will be continuous, and won't depend on the material in the cavity (compare with
emission spectrum). By a
theorem proved by Kirchhoff, this curve depends
only on the
temperature of the cavity walls.
Calculating this curve was a major challenge in theoretical physics during the late nineteenth century. The problem was finally solved in 1901 by
Max Planck as
Planck's law of black-body radiation.
By making changes to
Wien's Radiation Law (not to be confused with
Wien's displacement law) consistent with
thermodynamics and
electromagnetism, he found a mathematical formula fitting the experimental data in a satisfactory way. To find a physical interpretation for this formula, Planck had then to assume that the energy of the oscillators in the cavity was quantized (for example, integer multiples of some quantity).
Einstein built on this idea and proposed the quantization of electromagnetic radiation itself in 1905 to explain the
photoelectric effect. These theoretical advances eventually resulted in the superseding of classical electromagnetism by
quantum electrodynamics. Today, these quanta are called
photons and the black-body cavity may be thought of as containing a
gas of photons. In addition, it led to the development of quantum probability distributions, called
Fermi-Dirac statistics and
Bose-Einstein statistics, each applicable to a different class of particle, which are used in quantum mechanics instead of the classical distributions.
See also fermion and boson.
The wavelength at which the radiation is strongest is given by
Wien's displacement law, and the overall power emitted per unit area is given by the
Stefan-Boltzmann law. So, as temperature increases, the glow color changes from red to yellow to white to blue. Even as the peak wavelength moves into the ultra-violet, enough radiation continues to be emitted in the blue wavelengths that the body will continue to appear blue. It will never become invisible — indeed, the radiation of visible light increases
monotonically with temperature.
The
radiance or observed intensity isn't a function of direction. Therefore a black body is a perfect
Lambertian radiator.
Real objects never behave as full-ideal black bodies, and instead the emitted radiation at a given frequency is a fraction of what the ideal emission would be. The
emissivity of a material specifies how well a real body radiates energy as compared with a black body. This emissivity depends on factors such as temperature, emission angle, and wavelength. However, it's typical in engineering to assume that a surface's spectral emissivity and absorptivity don't depend on wavelength, so that the emissivity is a constant. This is known as the
grey body assumption.
Although Planck's formula predicts that a black body will radiate energy at all frequencies, the formula is only applicable when many photons are being measured. For example, a black body at room temperature (300 K) with one square meter of surface area will emit a photon in the visible range once every thousand years or so, meaning that for most practical purposes, the black body doesn't emit in the visible range.
When dealing with non-black surfaces, the deviations from ideal black-body behavior are determined by both the geometrical structure and the chemical composition, and follow
Kirchhoff's Law: emissivity equals absorptivity, so that an object that doesn't absorb all incident light will also emit less radiation than an ideal black body.
In
astronomy, objects such as
stars are frequently regarded as black bodies, though this is often a poor approximation. An almost perfect black-body spectrum is exhibited by the
cosmic microwave background radiation.
Hawking radiation is black-body radiation emitted by
black holes.
Equations governing black bodies
Planck's law of black-body radiation
» :
Here
v > 0 indicates a receding source, and
v < 0 indicates an approaching source.
This is an important effect in astronomy, where the velocities of stars and galaxies can reach significant fractions of
c. An example is found in the
cosmic microwave background radiation, which exhibits a dipole anisotropy from the Earth's motion relative to this blackbody radiation field.
Further Information
Get more info on 'Black Body'.
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