What is Black body: Definition and 159 Discussions

A black body or blackbody is an idealized physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence. The name "black body" is given because it absorbs all colors of light. A black body also emits black-body radiation. In contrast, a white body is one with a "rough surface that reflects all incident rays completely and uniformly in all directions."A black body in thermal equilibrium (that is, at a constant temperature) emits electromagnetic black-body radiation. The radiation is emitted according to Planck's law, meaning that it has a spectrum that is determined by the temperature alone (see figure at right), not by the body's shape or composition.
An ideal black body in thermal equilibrium has two notable properties:
It is an ideal emitter: at every frequency, it emits as much or more thermal radiative energy as any other body at the same temperature.
It is a diffuse emitter: measured per unit area perpendicular to the direction, the energy is radiated isotropically, independent of direction.An approximate realization of a black surface is a hole in the wall of a large insulated enclosure (an oven, for example). Any light entering the hole is reflected or absorbed at the internal surfaces of the body and is unlikely to re-emerge, making the hole a nearly perfect absorber. When the radiation confined in such an enclosure is in thermal equilibrium, the radiation emitted from the hole will be as great as from any body at that equilibrium temperature.Real materials emit energy at a fraction—called the emissivity—of black-body energy levels. By definition, a black body in thermal equilibrium has an emissivity ε = 1. A source with a lower emissivity, independent of frequency, is often referred to as a gray body.
Constructing black bodies with an emissivity as close to 1 as possible remains a topic of current interest.In astronomy, the radiation from stars and planets is sometimes characterized in terms of an effective temperature, the temperature of a black body that would emit the same total flux of electromagnetic energy.

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  1. Philip Koeck

    I Radiation from solid black bodies

    I understand that Planck's law is derived for a cavity with a hole in it. I haven't found a clear argument that the same result and all results that follow from it also apply to solid surfaces that are black. Can anybody point me to a text that shows this?
  2. sinus

    I The kinetic energy of proton-electron for a black body

    I'm watching a video about " What is a black body?". That video said when the light interacts with the surface of a body, the electron and proton start oscillating. The electrons gain more transferred energy from the light that became its kinetic energy, rather than the proton because its mass...
  3. snoopies622

    I Are Planck's black body oscillators QHOs?

    What is the connection between (1) Planck's quantum hypothesis that the oscillators of a blackbody can possess only discrete quantities of energy E=nhv n=1,2,3.. (2) the fact that the energy eigenvalues of the quantum harmonic oscillator wavefunction are also separated by intervals of hv...
  4. H

    I Is Planck's Law of Black-Body Accurate?

    First of all, Is ##\beta## given by the Planck's law of black-body is the amount of power contained in radiation emitted by a black body? I'm not sure to fully understand the law above. Does it means that if amount of power over all the frequencies is greater than the energy needed to remove an...
  5. Conn_coord

    I The CMB has a thermal black body spectrum

    https://en.wikipedia.org/wiki/Wien's_displacement_law "Maxima differ according to parameterization ... Using the value 4 to solve the implicit equation yields the peak in the spectral radiance density function expressed in the parameter radiance per proportional bandwidth. (That is, the density...
  6. LCSphysicist

    Black body radiation and gas

    The image above is the solution posted by the book. I can follow the reasoning that has been used, but i have a trouble particularly at the first equation itself. Why should $$n = 2 \rho / m_{H} $$ instead of $$n = \rho / (2m_{H})$$, since the mass os a molecule of hydrogen is two times the...
  7. V

    B Two questions about Black body radiation

    I have questions about Black body radiation. see the attached image 1). It explains that the Spectral radiance measurement of 7000K temperature is the same as both 300 and 520 nm wavelength light. See here both A and B shows 60. Is my understanding correct? 2). Is spectral radiance the number...
  8. A

    I Black body radiation -- Some basic questions to aid my understanding

    Hi I am currently taking a physics course and studying black body radiation.I have already seen a good number of books , but I have a lot of unanswered questions. -What does the black body radiation, which is approximately the radiation of the sun, has to do with standing waves inside a cavity...
  9. H

    B Spinning Black Hole Drags Space-Time: What Causes Friction?

    If a Black Hole is spinning (perhaps they all do) I have heard it distorts the 'fabric' of Space-time in the vicinity. What is the 'friction' component which allows the distortion?
  10. E

    Black body radiation -- Spherical shell surrounding a star

    I don't understand how this can be solved. The official solution was: F=\sigma*T^4 E=F*4\pi R^2*60*60 This doesn't make sense to me, as it seems to imply that the energy that the black body radiates depends on the radius of the shell. For a very large shell the body will reflect...
  11. jjson775

    Power of black body radiation at 1.00 nm

  12. naviakam

    Black body radiation vs electric discharge in a gas

    Black body radiation formula contains power and exponential terms. Electric discharge in a gas results in the ion acceleration; the ion distribution comprises power and exponential terms too. Any connection between these two phenomena (i.e. black body and potential) could be established?
  13. LCSphysicist

    Black Body Entropy: Solving the Puzzle of q and N

    I thought that would be something like, using similar counts from Einstein solid, ##S = kln(\frac{(q+N-1)!}{q!(N-1)!})## Where q is ##hv## v is frequency But the results are not similar, so i am little stuck
  14. LCSphysicist

    Black Body Radiation: Facts & Overview

    I am really stuck at this question, i would appreciate any tips you have. I already try to goes with series but the calculation is getting harder and tiring, so probably this is not the right way.
  15. carter7gindenv

    I Why is the black body spectrum bell shaped if it's absorptance is equal to 1?

    This bother me too much. A black body is said to absorb every incoming radiation. Looking at absorptance we have : A=Φabs/Φreceived So if a Black body absorbs everything we have A=1 Φ is a flux meaning object/time. This means that whatever the wavelength a black body will absorb the maximal...
  16. SebastianRM

    Black Body Radiation -- why is it not at discrete wavelengths?

    I was looking at Kirchoffs Laws: "A solid, liquid or dense gas produces a continuous spectrum". I would expect objects to produce an emission spectrum since we would be observing the photons that come from spontaneous emission of electrons in excited states. This photons are specific to the...
  17. D

    Black body radiation and the derivation of Stefan Boltzman

  18. Cocoleia

    Black Body radiation and Planck's radiation distribution

    I am quite confused, as I start this question. I can easily find the following when searching up Planck's law: However, this is not u. My prof is quite unclear and sometimes chooses his own variables as he sees fit, so i am not sure if this would be equivalent to what he is looking for u(λ)dλ...
  19. J

    I Relation between spectral radiance and density of cavity

    Hi, my teacher showed us how we can derive de relation between spectral radiance and density of cavity (of a black hole), but I have a doubt. This is the equation of the energy that are coming from definited directions by the intervals of angles θ and Φ with frequency in a determined interval...
  20. Max Loo Pin Mok

    Black body radiation, Planck's law, derivation

    The following are 3 equations of Planck's law or Planck's distribution function. Are they all correct? How do they derive from each other? Equation One: From page 512 of http://metronu.ulb.ac.be/npauly/art_2014_2015/shockley_1961.pdf We denote by Qs the number of quanta of frequency greater...
  21. Krushnaraj Pandya

    Calculating Total Power Emitted by a Black Body in a Spherical Chamber

    Homework Statement A spherical body is enclosed in a spherical chamber which acts like a perfectly black body. The reflectance of the body is 0.4 and transmittance is negligible, the temperature of the body and surrounding temperature is constant at T Kelvin. The total power that comes out of...
  22. A

    I Radiance and energy density of a black body

    How can I find the relation between the radiance and the energy density of a black body? According to Planck's law, the energy density inside a blackbody cavity for modes with frequency ##\nu \in [\nu, \nu + \mathrm{d}\nu]## is given by $$ \rho(\nu, T)\mathrm{d}\nu =...
  23. YoungPhysicist

    I Is the Sun a Perfect Black Body? Investigating the Reflection of Radiation

    Black bodies are objects that don't reflect radiation. But If I shoot light beams to the sun, it doesn't reflect it , so does that mean the sun is a black body(or at least very close to a theoretical one)?
  24. D

    Temperature as a function of time in black body radiation

    Homework Statement 2. Consider a metal sphere of radius R and heat capacity C, initially at a temperature To which is much hotter than the background temperature. a) Derive an analytical result for the temperature of such a sphere as a function of time. Clearly state any simplifying...
  25. B

    B Photon mean free path (regarding CMB black body assumption)

    Per Wikipedia (Outer Space) referencing Davies, P. C. W. (1977), "...the mean free path of a photon in intergalactic space is about 10E23 km, or 10 billion light years." Per Lawrence Krauss (1999), it is longer than the size of the visible universe. What is the current thinking about this?
  26. EF17xx

    Understanding Perfectly Black Bodies

    Hi! I'm trying to understand a perfectly black body. So the definition I have found is that a black body is one that absorbs radiation of ALL wavelengths and reflects NONE. Therefore it appears black at low temperatures. And when heated it emits radiation of all wavelengths making it appear...
  27. A

    I Does the color of an object change as it gets hotter?

    I'm talking hotter than T = 6000k. The higher the temperature, the more the curve in the attached figure would shift to the left (while at the same time getting higher). So the intensity peak would eventually fall back into the invisible portion (very small wavelength this time) of the...
  28. Sophrosyne

    I Understanding black body radiation

    The field of quantum mechanics was launched with scientists struggling to understand blackbody radiation. My question is: what is the source of this observed radiation? Is it the nuclei in the matrix of the metal jiggling around ever more energetically as you heat up the metal? Or is it the...
  29. CHANDRABHAN

    Can black body retain it`s absorbed heat energy?

    Can black body retain it`s absorbed heat energy? If yes, then why we can not convert this energy into a usefull form such as into electric energy as solar do?
  30. patric44

    Quantum Need a book to explain the princple of quanization

    hi guys i am struggling to understand how and why quantization of energy solves the UV catastrophe and the black body problem ? and how they get to the Rayleigh - jeans equation in the first place ? and why plank modified the equation the way he did ? and why should the harmonic oscillators...
  31. F

    I Black Body and the colored objects around us

    Hello Forum, A quick discussion about blackbody theory. As a premise, this is what I know: A blackbody is an ideal body that absorbs all types of radiation incident on it, no matter its wavelength. Once absorbed, the energy is used to increase the blackbody's temperature and when the...
  32. D

    B Gravastar vs. Neutron Stars: A New Theory for Black Hole Formation?

    Has anyone else heard of that gravity waves may be the result of another type of supernova remains, called a Grav-Star? It seems almost to physically mimic a neuron star but stopped just shy of becoming a black hole, yet it still has enough of a gravity well to prevent light from escaping. Is...
  33. Pushoam

    Ratio of energy densities of black body radiation

    Homework Statement Homework EquationsThe Attempt at a SolutionThe energy density is given as ## u = \frac { 8 \pi {\nu }^2}{c^3}~ \frac { h \nu} {e^{ \frac { h\nu}{k_B T}} – 1}.## EDIT : I put the constant C. ## \frac { u( 2 \nu) } {u(\nu)} = C \frac { {e^{ \frac { h\nu}{k_B T}} – 1}...
  34. kal

    I Trouble understanding the idea of a cavity radiator being a Black Body

    I have been trying to understand the role of a cavity as a black body radiator in the derivation of planks black body radiation law but it has left me with 5 main questions: 1. If an object is a perfect absorber it must also be a perfect emitter, meaning that (allowing for a cavity not being a...
  35. R

    B Understanding Black Body Radiation and the Sun's Color Temperature

    So, if frequency(max) of light emitted from an object proportional to temperature in kelvin, how can sun have max frequency around the yellow region while blue flames are much less hot?
  36. N

    Bulbs that only emit long IR (>1200nm) or 1000K color temp.

    I am looking for bulbs that would be emitting a spectrum characterized by a color temperature of between 500K and 1000K or any kind of light fixture that only emits long IR wavelengths. Does such a thing exist?
  37. D

    Expressing entropy of black body radiation

    Homework Statement By applying the first law to a quasi static process, show that the entropy can be expressed as S = (16σ/3c) VT3 Homework Equations U = 4(σ/c) VT4 PV = 1/3 U[/B]The Attempt at a Solution I know I should be using dS = dQ/T but it's unclear to me how to use this unless I...
  38. K

    Calculating Radiation Power of a Ball at 10000K in the Range of 400nm-800nm

    Homework Statement I need to calculate radiation power of ball in range 400nm-800nm. T=10000K d=1um. Homework Equations I think I need to use Planck equation and integrate that. My equation in link: http://imgur.com/XbAUwJ8 The Attempt at a Solution I look for equations and laws and try to do...
  39. K

    The Sun treated as a perfect Black Body

    Homework Statement At lunch, the Sun's thermal energy incident on the surface of the Earth is 1.4 kW/m^2. Given the radius of the Sun, R, distance from Earth, r, and treating the Sun like a perfect black body, calculate the total intensity of its radiation and determine its temperature...
  40. I

    I Black body, angle of incidence

    I'm trying to understand the solution to an exercise. The solution claims that that the absorbed power of a black body from sunlight with an angle of incidence of ##30^\circ## is ##P=P_0\cos(30^\circ)## The reasoning here being to take the component orthogonal to the surface. However what...
  41. K

    Why does Planck's BB emissive power increase with n?

    This question is regarding the dependence of Planck's law for black-body (BB) radiation intensity (or integrating over a hemisphere, the emissive power, E = pi * I). Physically speaking, why is it that a BB emitting in a medium with n>1 (n being index of refraction) emits a higher power/area...
  42. Matthew Adams

    I Emission spectroscopy of an acetylene flame

    Hey there folks! This is my first post so please be gentle... ;-) One of my students has been studying the emission spectrum of an acetylene flame. A gas mixture from both an acetylene cannister & an oxygen canister is fed out to a nozzle, a flame is ignited and we study the emission...
  43. A

    I How to calculate the energy emitted through IR waves

    The energy emitted by a body in watts/m2 is = εσT4. In the case of a perfect black body, ε=1. If the body only emits IR light, what should be the value of ε?
  44. E

    Radiation of a Spherical Body Inside a Black Body

    Homework Statement A spherical body of area A and emissivity 0.6 is kept inside a perfectly black body. Total heat radiated by the body at temperature T is (A) 0.4σAT4 (B) 0.8σAT4 (C) 0.6σAT4 (D) 1.0σAT4 Homework Equations Stefan-Boltzmann's Law: P = AεσT4The Attempt at a Solution If it wasn't...
  45. F

    I Why do black bodies get hotter than white bodies in the sun?

    Hello Forum, A black body is an object with emissivity=1 that absorbs as well as it emits. Does that mean that its temperature T stops rising after it reaches thermal equilibrium but it rises before then since absorption dominates over emission? What about a white body? That would be a perfect...
  46. mukul

    Black Body Radiation and ambient temperature

    Homework Statement In a dark room with ambient temperature T0, a black body is kept at a temperature T. Keeping the temperature of the black body constant (at T), sunrays are allowed to fall on the black body through a hole in the roof of the dark room. Assuming that there is no change in the...
  47. Ontophobe

    Black Body Radiation and Kinetic Energy

    I see that black body radiation slowly depletes a body's thermal energy, which is just another way of saying that black body radiation depletes the kinetic energy of a body's constituent particles. But does black body radiation also cut into the kinetic energy of the body as a whole, such that a...
  48. J

    Why does a black body radiate in all the frequency spectrum?

    I understand why a black body absorbs every frequency(it is the definition of a black body!) but i do not understand why it also radiates at all frequency spectrum.
  49. J

    Black Body radiation and thermal equilibrium

    Wikipedia writes: "Black-body radiation is the type of electromagnetic radiation within or surrounding a body in thermodynamic equilibrium with its environment". Why does it write "thermodynamic equilibrium"? If it is not in a thermodynamic equilibrium, then what changes as far as the absorption...
  50. F

    O Physics: Bose-Einstein Condensates and Laser Cooling

    Heat is a type of energy that is transferable and increases thermal energy. Thermal energy is being released as infrared radiation hence the term heat vision. Does that mean heat is capable of producing so called black body radiation here. Is the infrared radiation caused by the photon emission...
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