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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 is 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 will 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.
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.
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 is nearly certain to be absorbed. This occurs regardless of the wavelength of the radiation entering (as long as it is 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 (i.e., the amount of light emitted from the hole at each wavelength) will be continuous, and will not 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 (i.e., 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.
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 is typical in engineering to assume that a surface's spectral emissivity and absorptivity do not 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 does not 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 does not 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
- Main article: Planck's law
 Wien's displacement law
- Main article: Wien's displacement law
The relationship between the temperature T of a black body, and wavelength λmax at which the intensity of the radiation it produces is at a maximum is
 Stefan–Boltzmann law
- Main article: Stefan–Boltzmann law
 Radiation emitted by a human body
|Much of a person's energy is radiated away in the form of infrared energy. Some materials are transparent to infrared light, while opaque to visible light (note the plastic bag). Other materials are transparent to visible light, while opaque or reflective to the infrared (note the man's eyeglasses).|
Black-body laws can be applied to human beings. For example, some of a person's energy is radiated away in the form of electromagnetic radiation, most of which is infrared.
The net power radiated is the difference between the power emitted and the power absorbed:
- Pnet = Pemit − Pabsorb.
Applying the Stefan–Boltzmann law,
The total surface area of an adult is about 2 m², and the mid- and far-infrared emissivity of skin and most clothing is near unity, as it is for most nonmetallic surfaces. Skin temperature is about 33°C, but clothing reduces the surface temperature to about 28°C when the ambient temperature is 20°C. Hence, the net radiative heat loss is about
The total energy radiated in one day is about 9 MJ (Mega joules), or 2000 kcal (food calories). Basal metabolic rate for a 40-year-old male is about 35 kcal/(m²·h), which is equivalent to 1700 kcal per day assuming the same 2 m² area. However, the mean metabolic rate of sedentary adults is about 50% to 70% greater than their basal rate.
There are other important thermal loss mechanisms, including convection and evaporation. Conduction is negligible since the Nusselt number is much greater than unity. Evaporation (perspiration) is only required if radiation and convection are insufficient to maintain a steady state temperature. Free convection rates are comparable, albeit somewhat lower, than radiative rates. Thus, radiation accounts for about 2/3 of thermal energy loss in cool, still air. Given the approximate nature of many of the assumptions, this can only be taken as a crude estimate. Ambient air motion, causing forced convection, or evaporation reduces the relative importance of radiation as a thermal loss mechanism.
Also, applying Wien's Law to humans, one finds that the peak wavelength of light emitted by a person is
This is why thermal imaging devices designed for human subjects are most sensitive to 7–14 micrometers wavelength.
 Temperature relation between a planet and its star
The surface temperature of a planet depends on a few factors:
- Incident radiation (from the Sun, for example)
- Emitted radiation (for example Earth's infrared glow)
- The albedo effect (the fraction of light a planet reflects)
- The greenhouse effect (for planets with an atmosphere)
- Energy generated internally by a planet itself (due to Radioactive decay, tidal heating and adiabatic contraction due to cooling).
For the inner planets, incident and emitted radiation have the most significant impact on surface temperature. This derivation is concerned mainly with that.
If we assume the following:
- The Sun and the Earth both radiate as spherical black bodies.
- The Earth is in thermal equilibrium.
- The Earth absorbs all the solar energy that it intercepts from the Sun.
then we can derive a formula for the relationship between the Earth's surface temperature and the Sun's surface temperature.
- is the Stefan–boltzmann constant,
- is the surface temperature of the Sun, and
- is the radius of the Sun.
The Sun emits that power equally in all directions. Because of this, the Earth is hit with only a tiny fraction of it. This is the power from the Sun that the Earth absorbs:
- is the radius of the Earth and
- is the distance between the Sun and the Earth.
Even though the earth only absorbs as a circular area πR2, it emits equally in all directions as a sphere:
- where TE is the surface temperature of the earth.
Now, in the first assumption the earth is in thermal equilibrium, so the power absorbed must equal the power emitted:
- So plug in equations 1, 2, and 3 into this and we get
Many factors cancel from both sides and this equation can be greatly simplified.
 The result
After canceling of factors, the final result is
where is the surface temperature of the Sun, is the radius of the Sun, is the distance between the Sun and the Earth, and is the average surface temperature of the Earth.
In other words, given the assumptions made, the temperature of the Earth depends only on the surface temperature of the Sun, the radius of the Sun, and the distance between the Earth and the Sun.
 Temperature of the Sun
If we substitute in the measured values for Earth,
we'll find the effective temperature of the Sun to be
This is within three percent of the standard measure of 5780 kelvins which makes the formula valid for most scientific and engineering applications.
 Doppler effect for a moving blackbody
The Doppler effect is the well known phenomenon describing how observed frequencies of light are "shifted" when a light source is moving relative to the observer. If f is the emitted frequency of a monochromatic light source, it will appear to have frequency f' if it is moving relative to the observer :
where v is the velocity of the source in the observer's rest frame, θ is the angle between the velocity vector and the the observer-source direction, and c is the speed of light. This is the fully relativistic formula, and can be simplified for the special cases of objects moving directly towards ( θ = π) or away ( θ = 0) from the observer, and for speeds much less than c.
To calculate the spectrum of a moving blackbody, then, it seems straightforward to simply apply this formula to each frequency of the blackbody spectrum. However, simply scaling each frequency like this is not enough. We also have to account for the finite size of the viewing aperture, because the solid angle receiving the light also undergoes a Lorentz transformation. (We can subsequently allow the aperture to be arbitrarily small, and the source arbitrarily far, but this cannot be ignored at the outset.) When this effect is included, it is found that a blackbody at temperature T that is receding with velocity v appears to have a spectrum identical to a stationary blackbody at temperature T' , given by: 
For the case of a source moving directly towards or away from the observer, this reduces to
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.
 See also
- Effective temperature
- Color temperature
- Infrared thermometer
- Photon polarization
- Ultraviolet catastrophe
- Rayleigh-Jeans law
- ^ When used as a compound adjective, the term is typically hyphenated, as in "black-body radiation", or combined into one word, as in "blackbody radiation". The hyphenated and one-word forms should not generally be used as nouns.
- ^ Huang, Kerson (1967). Statistical Mechanics. New York: John Wiley & Sons.
- ^ Planck, Max (1901). "On the Law of Distribution of Energy in the Normal Spectrum" (HTML). Annalen der Physik 4: 553.
- ^ Landau, L. D.; E. M. Lifshitz (1996). Statistical Physics, 3rd Edition Part 1, Oxford: Butterworth-Heinemann.
- ^ Infrared Services. Emissivity Values for Common Materials. Retrieved on 2007-06-24.
- ^ Omega Engineering. Emissivity of Common Materials. Retrieved on 2007-06-24.
- ^ Farzana, Abanty (2001). Temperature of a Healthy Human (Skin Temperature). The Physics Factbook. Retrieved on 2007-06-24.
- ^ Lee, B.. Theoretical Prediction and Measurement of the Fabric Surface Apparent Temperature in a Simulated Man/Fabric/Environment System. Retrieved on 2007-06-24.
- ^ Harris J, Benedict F (1918). "A Biometric Study of Human Basal Metabolism.". Proc Natl Acad Sci U S A 4 (12): 370-3. PMID 16576330.
- ^ Levine, J (2004). "Nonexercise activity thermogenesis (NEAT): environment and biology". Am J Physiol Endocrinol Metab 286: E675-E685.
- ^ DrPhysics.com. Heat Transfer and the Human Body. Retrieved on 2007-06-24.
- ^ Cole, George H. A.; Woolfson, Michael M. (2002). Planetary Science: The Science of Planets Around Stars (1st ed.). Institute of Physics Publishing, 36–37, 380–382. ISBN 0-7503-0815-X.
- ^ The Doppler Effect, T. P. Gill, Logos Press, 1965
- ^ McKinley, John M., "Relativistic transformations of light power", Am. J. Phys. 47 (7), Jul 1979
 Other textbooks
- Kroemer, Herbert; Kittel, Charles (1980). Thermal Physics (2nd ed.). W. H. Freeman Company. ISBN 0-7167-1088-9.
- Tipler, Paul; Llewellyn, Ralph (2002). Modern Physics (4th ed.). W. H. Freeman. ISBN 0-7167-4345-0.
 External links
- Calculating Blackbody Radiation Interactive calculator with Doppler Effect. Includes most systems of units.
- Cooling Mechanisms for Human Body - From Hyperphysics
- Descriptions of radiation emitted by many different objects
- BlackBody Emission Applet
- "Blackbody Spectrum" by Jeff Bryant, The Wolfram Demonstrations Project, 2007.