What is Blackbody radiation: Definition and 113 Discussions

Black-body radiation is the thermal electromagnetic radiation within or surrounding a body in thermodynamic equilibrium with its environment, emitted by a black body (an idealized opaque, non-reflective body). It has a specific spectrum of wavelengths, inversely related to intensity that depend only on the body's temperature, which is assumed for the sake of calculations and theory to be uniform and constant.The thermal radiation spontaneously emitted by many ordinary objects can be approximated as black-body radiation. A perfectly insulated enclosure that is in thermal equilibrium internally contains black-body radiation and will emit it through a hole made in its wall, provided the hole is small enough to have a negligible effect upon the equilibrium.
In a dark room, a black body at room temperature appears black because most of the energy it radiates is in the infrared spectrum and cannot be perceived by the human eye. Since the human eye cannot perceive light waves below the visible frequency, a black body at the lowest just faintly visible temperature subjectively appears grey, even though its objective physical spectrum peak is in the infrared range. The human eye essentially does not perceive color at low light levels. When the object becomes a little hotter, it appears dull red. As its temperature increases further it becomes bright red, orange, yellow, white, and ultimately blue-white.
Although planets and stars are neither in thermal equilibrium with their surroundings nor perfect black bodies, black-body radiation is used as a first approximation for the energy they emit. Black holes are near-perfect black bodies, in the sense that they absorb all the radiation that falls on them. It has been proposed that they emit black-body radiation (called Hawking radiation), with a temperature that depends on the mass of the black hole.The term black body was introduced by Gustav Kirchhoff in 1860. Black-body radiation is also called thermal radiation, cavity radiation, complete radiation or temperature radiation.

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  1. phyTens

    I How is there any pressure at all in isotropic radiation?

    It is a standard result that in a blackbody radiation there is a pressure (at a certain frequency), given by $$P_\nu=\frac 1 3 U_\nu$$ However, I am quite confused by this result. Firstly, how do we even define pressure in radiation gas? I would think that this would be the pressure on a small...
  2. PainterGuy

    I Blackbody radiation and the cosmic microwave background

    Hi, The following is my basic understanding of blackbody radiation spectrum. The important sections are in boldface. Source: https://en.wikipedia.org/wiki/Black-body_radiation#Spectrum Question 1: The quote above says that at room temperature (let's say 20 C or 293 K) the emission is in the...
  3. B

    Interpreting Weird Results: J_u, P_1 and T_1

    I need someone to check my work, because I'm getting weird results that I'm not able to interpret physically for parts b and c. Thanks in advance. For part a... ##J_u = e_1 \sigma_B T^4## ##P_1 = AJ_u = e_1 \sigma_B AT_1^4## ## T_1 = \left( \frac {P_1} {e_1 \sigma_B A} \right)^{\frac 1 4} ##...
  4. M

    I Blackbody radiation in quantum mechanics

    Hello! If I place a particle with more energy levels (of the order of kT) in a well defined state, in a thermal bath at temperature T, how will the blackbody radiation affect the internal state of the particle i.e. will the distribution be classical or QM? Basically, if I prepare that particle...
  5. I

    I Relation between blackbody radiation and spontaneous emission

    I'm wondering what the relationship between blackbody radiation and spontaneous emission is. As far as I know, there are three sources of EM radiation - thermal radiation, oscillating dipole (multipole?), and LASER. And it seems like light emission from an atom can be separated into two...
  6. A

    Exploring the Relationship between CMB Radiation and Blackbody Temperature

    Here is something that struck a note to me, they give the CMB radiation in it's frequency which is in Ghz as the name "microwave" implies and then they also give a temperature in Kelvin. But how can light aka EM radiation have a temperature? I thought only matter with mass can have a temperature...
  7. tanaygupta2000

    Molar Specific heat of Blackbody radiation

    For a body at temperature T, the radiative energy per unit area E depends on 4th power of T. I can obtain expression for specific heat c by differentiating Stefan's law with respect to T. Would it be the correct way of approaching this problem? Or do I need to employ certain models from Solid...
  8. R

    I A qualitative question about Blackbody Radiation

    would this be a correct understanding of blackckbody radiation phenomena? in particular the intensity versus wavelength curve? "A Blackbody consists of oscillators of molecular dimensions. Intensity is proportional to number of oscillators with sufficient energy hv emitting radiation, and that...
  9. Viona

    I Blackbody Radiation: Questions Answered on Causes & Oscillators

    Hi every one I have three simple questions: what causes Blackbody radiation? what does Planck meant by the oscillators? why the Blackbody radiation is continuous while the emission spectra of atoms (gases) is discrete? I know that the discrete emission spectra of atoms was explained by Bohr's...
  10. K

    I Why is a resonant frequency assumed in blackbody radiation?

    Hello! In the descriptions of the BB radiation that I read, I see that we assume we have a cavity at a fixed temperature in equilibrium, we make a hole in it and we look at the intensity of different frequencies emitted through that hole. As far as I understand, the intensity dependence on the...
  11. JulioHC

    Derivation of blackbody radiation equations for stars

    Good evening, As part of my course, I had this week two lectures about the blackbody radiation and its relation to the stars. While I do understand how to use results such as the Stefan-Boltzmann law and Wien's Law I'm lost in other parts. I think the only parts that I don't understand yet are...
  12. 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...
  13. nmsurobert

    I Are Stars Absorbing and Re-Emitting Radiation from Space?

    Ive been reading about it for a while and I'm still confused. i understand that a star is not a perfect blackbody. but do stars absorb radiation from space and then re-emit that energy into space?
  14. Arup Biswas

    Blackbody Radiation: Explained!

    When I study any book of Quantum Mechanics like Resnick or Beiser etc all start with blackbody radiation! But how this radiation is produced? Google says due to increased collision of particles causing the acceleration and em wave but what particles? How they are accelerated from what? Like if...
  15. T

    Why is blackbody radiation continuous?

    Plasmas can emit radiation based on the acceleration of charged particles (which we generally consider as continuous), but for un-ionized matter compounds, transitions are quantized and photons have particular energies. At room temperature, collisional excitations are typically dominant. But if...
  16. Ron19932017

    I How to approach a more perfect blackbody?

    I wonder how an object, like our sun, can approach a more perfect blackbody. We know that by the wiki definition, blackbody is something that absorb all radiation and is in thermal equilibrium. Its spectrum only depends on T. We also know that, our Sun's spectrum is blackbody like, while a...
  17. J

    B Blackbody Radiation: Explaining Ultraviolet Catastrophe & Planck's Solution

    Hello, I'm an English student and external candidate, hoping to take my Physics with me through life. I have some questions regarding a topic I'm researching, currently. I have a book "Advanced Physics - Steve Adams, Jonathan Allday", which details 'Blackbody Radiation', as evidence for a...
  18. henry wang

    How does the surface property affect blackbody radiation?

    In an attempt to explain why a matt surface of aluminium is a better emitter/absorber of blackbody radiation than shiny surface of aluminium, my university lecturer suggested to me that: By brushing a metal surface to create a matt finish, the surface of the metal becomes rougher. Rougher means...
  19. Samama Fahim

    Spectral Intensity as a Function of Wavelength in Blackbody Radiation

    A blackbody is also a perfect emitter giving off electromagnetic waves at all frequencies. A detector could measure the intensity of the radiation it receives through the prism. By moving the detector to different positions, you could measure the intensity of light as a function of color or...
  20. Edward258

    Find the temperature increase of a rotating disk

    Homework Statement It's a Blackbody radiation problem: A beam of wavelength λ, in the state of right circular polarization, leads to an absorbent disk.The mass of the disk is m, it's specific heat is C, and its moment of inertia is I .The disk is initially at rest, but after a lapse of time...
  21. G

    I Discrete vs Continuous Spectra in Blackbody Radiation?

    I was reading this article which talks about the theoretical model behind blackbody spectra: http://www.cv.nrao.edu/course/astr534/BlackBodyRad.html At the start, it mentions standing waves in a cavity. Standing waves in this model consist of an integer number of wavelengths. The standing waves...
  22. T

    Blackbody Radiation - Peak wavelength

    Homework Statement The Planck blackbody spectrum is given by u(ω,t)=\frac{ħω^3}{π^2c^3(e^{βħω}-1)} Show that the peak of the Planck spectrum for a blackbody at a temperature T occurs at the wavelength λ_{max}T=0.29 where T is in Kelvin and λmax is in cm. Homework Equations...
  23. Neha98

    Verify particle nature of electron using blackbody radiation

    It's mentioned in my book that blackbody radiation can verify the particle nature of electron could anyone explain this to me,please?
  24. T

    Converting between λ and ν for Blackbody Radiation?

    Forgive me for this stupid question, but how do I convert between and I tried c = νλ but that doesn't work. This is the Rayleigh Jeans Law by the way.
  25. S

    Jansky for blackbody radiation

    Hi! I hope this is the right place for this question. I have a plot of mJy vs frequency and I want to fit a blackbody plot to it. I just don't know how to convert the y-axis correctly to use the formula for the fit. Any help is appreciated.
  26. G

    Blackbody Radiation: Ideal Emitters at Room Temperature

    Hi, Very basic question. Blackbodies are ideal emitters: at every frequency, they emit an amount of energy equal to or greater than any other object at the same temperature. Furthermore, they were named blackbodies since, AT ROOM TEMPERATURE, i.e. 300K, whatever radiation they re-emit (after...
  27. Hyo X

    KT (energy) vs Blackbody Radiation for molecule in a box

    I'm trying to understand the meaning of kT (energy) in molecular systems, how to define the temperature of an individual molecule, and how a molecule receives thermal energy or dissipates thermal energy. Here is my 'gendanken': One molecule is floating in a box, in vacuum. The temperature of...
  28. A

    Confusion With Blackbody Radiation

    A blackbody is a theoretical object that perfectly absorbs all the light that falls on it. From what I understand this is an ideal situation and does not actually exist in reality. Certain objects are close to being a blackbody but they do not absorb 100% of the light that hits it (i.e. some...
  29. Raman Choudhary

    Understanding Blackbody Radiation in Quantum Mechanics

    I'm trying to start understanding quantum mechanics, and the first thing I've come across that needs to be understood are black bodies. But I've hit a roadblock at the very first paragraphs. :( According toWikipedia: A black body (also, blackbody) is an idealized physical body that absorbs all...
  30. A

    How Do You Calculate Total Energy in Blackbody Radiation?

    Homework Statement Show that the total energy of the radiation in a volume V at temperature T is Hint: Homework Equations The Attempt at a Solution The hint doesn't make sense to me, and those are the equation that I found to be perhaps relevant. Do I integrate the second equation? I'm...
  31. E

    Blackbody Radiation and the Inverse Square Law

    I am currently confused with the concept of the blackbody radiation and the inverse square law. Planck's function for the radiation of a blackbody is in ##W sr^{-1} m^{-3} ##, is this somehow a form of intensity (because of the watts per square meter unit)? If it does, doesn't intensity...
  32. E

    Still Water as an Approximate Blackbody

    Can the temperature of a still water be calculated using Planck's Blackbody Radiation? For instance, I have the intensity image of water (of course, this is considered still, as it seems that it is currently stopped in time), and I want to calculate the temperature of the water, is it possible...
  33. T

    Blackbody Radiation: Max Intensity & Freq, Invisibility in UV Region?

    The black body curve of intensity vs frequency increases, reaches a maximum value and then decreases. As the temperature increases, the peak of the curve shifts to lower wavelengths or higher frequencies. 1) I only vaguely understand the relation between the nature of this graph and the fact...
  34. W

    Planck's Law vs Rayleigh-Jeans Law (Blackbody radiation)

    First time here, and looking for help on this. The 2nd part of this problem, I have seen some posts on and am still reviewing, but haven't found much on the 1st part. Homework Statement 1) Use l'Hopital's Rule to show that $${\lim_{\lambda\rightarrow 0^{+}}=0}\text{ and...
  35. CassiopeiaA

    Blackbody radiation over whole frequency spectrum

    I was studying black body radiation and how quantization of energy solves the problem of ultraviolet catastrophe. But I have a very fundamental doubt. A black body can be assumed as a cavity with a small hole with radiation leaking out of it. As the temperature of the black body is increased we...
  36. B

    Blackbody Radiation: QFT-Book Analysis

    Hello, in Schwartz's QFT-book it says that: "Classically, a box of size L supports standing electromagnetic waves with angular frequencies \omega_n = \frac{2\pi}{L}\left|\vec{n}\right|c (...)" I wonder if the factor 2 is really correct, I only get this factor 2 if I suppose that eg. for...
  37. D

    Power for a blackbody radiation

    If a blackbody is in equilibrium with the surrounding electromagnetic field, the power emitted by the surface of the blackbody will be related to the energy density of the electromagnetic field by P=\frac{cu}{4}. Try as I might, I haven't found a good derivation for this equation (the...
  38. Entanglement

    Evidence of photon existence and Blackbody Radiation

    How does the blackbody radiation prove the existence of photons or quanta as Planck described it, I've understood how the photoelectric effect proves the existence of photons, but the blackbody radiation seems quite vague to me. I would like a basic explanation for this, thanks in advance.
  39. D

    Blackbody Radiation: Exploring Beyond \nu

    I have a quick question about blackbody radiation. Planck modeled a blackbody as a collection of harmonic oscillators. Then he assumed that each oscillator could only have a energy E equal to nh\nu, where \nu is the frequency of the oscillator. My question is, how can a collection of...
  40. A

    Calculating Quanta Emitted by a Blackbody at 500 C

    Homework Statement The temperature of a blackbody is 500 C. If the intensity of the emitted radiation, 2.0 W/m^2, were due entirely to the most intense frequency component, how many quanta of radiation would be emiteed per second per square meter? Homework Equations λmaxT = 2.90 x...
  41. Matt atkinson

    Statistical Physics - blackbody radiation

    Homework Statement A cavity contains black body radiation at temperature at T=500K. Consider a optical mode in the cavity with frequency w=2.5x10^(13) Hz. Calculate; (a)the probability of finding 0 photons in the mode. (b)the probability of finding 1 photon in the mode (c)the mean number of...
  42. S

    Energy Density of Blackbody Radiation

    Homework Statement \mbox{Let} \ p(< \nu_{0}) \mbox{be the total energy density of blackbody radiation in all frequencies less than} \ \nu_{0}, \mbox{where} \ h \nu_{0} << kT. \mbox{Derive an expression for} \ p (< \nu_{0})Homework Equations p(v) dv = \dfrac{8 \pi h} {c^3} \dfrac...
  43. L

    SpectraSuite & Blackbody Radiation

    Hello, me and my friend have been doing a lab where we are measuring the wavelengths of light emitted by a blackbody. We are using the OceanOptics 4000USB spectrascope with the SpectraSuite software. The only problem we are having is with the software, it plots the wavelength on the x-axis and...
  44. S

    Damped Electric Oscillator: Blackbody Radiation & Energy Conservation

    consider energy for a damped electric oscillator . ("f" indicates the dipole moment of the oscillator) in the absence of the damping force U= \frac{1}{2}kx^2 +1/2 (\frac{d^2x}{dt^2}) ^2 and the energy conservation tells us dU=0. but if there is damping force we get the following...
  45. R

    Blackbody radiation and Planck constant questions

    Ive been reading lately about the Planck constant and have been trying to learn more about it if possible. The way I understand it, the constant came about from experiments with black body radiation. Planck noticed that the experimental data fit the equation e=hf. My first question: how...
  46. K

    Blackbody radiation - Radiative flux derivation of the Stefan-Boltz.

    I am trying to resolve some long standing problems I have encountered with blackbody radiation. Namely, the derivation of the radiation energy flux equation $$J=\sigma_{B} T^4$$. I understand the derivation of the energy density of photons in "a box". $$U/V=const. T^4$$ I do not understand the...
  47. J

    Blackbody radiation and the Ultraviolet Catastrophe

    Hi, please could someone help me as I am struggling to understand the classical blackbody theory. I will briefly summarize what my textbook says. It says that if the classical model was correct then a tennis ball should be emitting radiation in mainly the ultraviolet region. The idea at the time...
  48. S

    Blackbody radiation - quantum to classical

    I have a question regarding the parameters that reduces the Planck distribution to the Rayleigh-Jeans distribution. According to the Planck distribution, the average energy in a unit volume in the \nu frequency mode of a blackbody radiation field is <U> = \frac{h\nu}{e \frac{h\nu}{KT} - 1}...
  49. S

    How does the spectral distribution of blackbody radiation relate to Wien's law?

    Homework Statement a) Show that for photons of frequency \nu and wavelength \lambda : 1) d\nu = - c d\lambda / \lambda^{2} 2) u(\lambda)d\lambda = - u(\nu)d\nu 3) u(\lambda)d\lambda = u(\nu) c d\lambda / \lambda^{2} b) Show that the Rayleigh-Jeans spectral distribution of blackbody...