Blackbody Definition and 194 Threads

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. bapowell

    Insights A Poor Man's CMB Primer. Part 2: Bumps on a Blackbody - Comments

    bapowell submitted a new PF Insights post A Poor Man's CMB Primer. Part 3: Bumps on a Blackbody Continue reading the Original PF Insights Post.
  2. 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...
  3. 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...
  4. 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...
  5. 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...
  6. J

    Black body re-emission of incident radiation

    I know that a black body is an idealized body that absorbs all the incident EM radiation. And I also know that the BB emits a spectrum of thermal radiation that depends on its temperature. What I cannot understand is the re-emission. For what I've read, when in thermal equilibrium, a BB absorbs...
  7. 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...
  8. A

    Understanding Real-World Emission Spectra: Are Solids Similar to Blackbodies?

    Hi all, I've got two questions about the emissions spectrum from solids. Question #1: I feel like I have a reasonable understanding of line absorption and emission spectrum of low density gases based on transitions of electrons between discrete allowed energy levels in a gas. I'm trying to...
  9. L

    Emissivity & internal blackbody

    Please confirm my understanding and inquiry.. in material without transmission... 100% = Emissivity + Reflectivity... when using thermal camera (I own one).. we have to adjust the emissivity In two objects that is both 30 Celsius.. one a near black object with emissivity nearly 1 (black body)...
  10. G

    Temperature, Kinetic Energy, Boltzmann Factors, and SHO

    I am confused about the following; where am I going wrong here? 1. (1/2)kT is defined as the average kinetic energy of the molecules of a substance at temperature T, right? 2. You can derive the Boltzmann distribution/Boltzmann factors using (1/2)kT as the kinetic energy, making an argument...
  11. H

    Blackbody emission in 2D coordinates

    The spectral radiance of a blackbody has units of W·sr-1·m-2·Hz-1. How do I deal with these units if I want to think about a 2D problem of radiation in Cartesian coordinates? I assume that instead of a sphere of emission (which would result in artificial decrease in intensity with the inverse...
  12. 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...
  13. 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...
  14. 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...
  15. 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.
  16. 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...
  17. gfd43tg

    Blackbody Rooftop Homework | Emissivity 0.9, Absorbtivity 0.8

    Homework Statement The black roof of a building has an emissivity of 0.9 and an absorbtivity of 0.8 for solar radiation. The sun beats down at midday with an intensity of 300 BTU / hr ft2. a) If the temperature of the air and the surroundings is 68 °F, if the wind velocity is negligible...
  18. 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...
  19. 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...
  20. X

    Something that should be extremely simple - Blackbody Curve

    Hello! I am having an issue with something that should be extremely simple. Essentially, all I am trying to do is plot the blackbody curve at 2000K in terms of the wavelength. The formula I am using can be seen here.. (Don't feel like typing it)...
  21. K

    What is the blackbody problem and the ultraviolet catastrophe?

    Hello! I'm trying to research about the history and development of Quantum theory, especially with regards to Max Planck's energy quanta. I don't understand what was the problem in classical physics that required Planck to formulate quantum theory/energy quanta, which seemed to have...
  22. 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...
  23. 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...
  24. 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...
  25. 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...
  26. 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...
  27. 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...
  28. 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}...
  29. 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...
  30. D

    Deriving the density of states for waves in a blackbody

    Homework Statement Hey all, I am having trouble following some of the notes that my professor posted with regards to waves inside a blackbody; here is what he posted: (the part in bold is what I am just not understanding) "Inside the blackbody box, we need for the position of the walls...
  31. J

    What is the quantum mechanics behind blackbody radiation?

    I can't seem to find a definitive answer to my questions surrounding this topic, every textbook I read is rather vague. When Max Planck scratched the surface of quantum mechanics he introduced the idea of quantisation of energy in blackbodies. Could someone please explain how exactly a blackbody...
  32. O

    Photon wavelength probability distribution for blackbody

    Hey everyone, This is my first time posting on PF! I want to model the photons ejected from a blackbody source at temperature T. The question I want answered is: given a photon is detected, what is the probability of the photon having a wavelength λ? This amounts to just attaining the...
  33. M

    Blackbody Radiation and Motion: Exploring the Dynamics of Inertial Frames

    I just confused myself with this idea, need someone to fix it please! Say a spherical black body temperature T is moving with velocity v in some direction through some medium with a lower temperature. In the sphere's rest frame this emission is isotropic, however, in the rest frame of the...
  34. C

    Blackbody Radiation - Entropy and Internal Energy

    Homework Statement Expression for the entropy and internal energy of black body radiation. Using the below relations: Homework Equations Total free energy for black body: $$ F = (k_b TV/\pi^2) \int k^2 ln[1-exp(-\hbar ck/k_b T)]dk $$ Relationship between partition function and internal...
  35. Bailey

    Compton scattering and collision within blackbody

    Homework Statement Consider a blackbody contain atoms that typically produce photons with energy of about 7eV. Estimate the number of photon-electron collision each photon undergoes before it escape the object. Assume the blackbody has a temperature of 1000K. Homework Equations λ2 - λ1...
  36. X

    Temperature derived from ratio of blackbody radiation

    Homework Statement Show that the ratio of the blackbody fluxes from a star at two different frequencies (i.e., a color) is measured, then, in principle, the surface temperature of the star can be derived, even if the star's solid angle on the sky is unknown (e.g., if it is too distant to be...
  37. A

    Blackbody and absorption problem

    Well, this question may seem elementary to you but I simply do not have a background for it. Now the question: Sun radiation resembles a blackbody at over 5000°K. A solar cell's temperature is around 300°K. If we neglect loss in the cell, its emission and absorption coefficient are equal...
  38. V

    A problem about blackbody radiation

    The Stefan–Boltzmann law : J = kT^4 where k is Stefan–Boltzmann constant. but for non black body ( or grey body) the equ becomes J = εkT^4 where 0<ε<1 can anyone explain y a grey body emit less radiation when compare to the black body at same T? It can found the ans in wiki or the...
  39. T

    Wavelengths of sunlight, blackbody radiators, Planck's law, CCT

    A blackbody radiator emits radiation across the entire radiation spectrum. The "temperature" of the blackbody radiator (measured in kelvin) can be directly calculated from the peak wavelength of its radiation using http://en.wikipedia.org/wiki/Wien's_displacement_law"]Wien's[/PLAIN]...
  40. H

    Blackbody emission and absorption

    I am trying to understand radiative transport of thermal energy in materials from first (or close to first) principles. I do not understand the systematics/statistics of how photons are emitted and absorbed in a medium. How is it that photons can be emitted at so many wavelengths and atoms can...
  41. A

    EM waves and blackbody radiation question

    Homework Statement A rectangular metal plate measures 0.20 m long and 0.3 m wide. The plate is heated to a temperature of 1,433 K by passing a current through it. Assuming that it behaves like a blackbody, how much power does the plate radiate under these conditions? Homework Equations...
  42. Graeme M

    Deriving average surface temperature of a blackbody sphere?

    I have been reading about climate change recently and I have run into a problem understanding the Earth's energy budget. I will try to summarise my question but am happy to try to expand if needed. Bear in mind I am just an interested average Joe with little science background and probably a...
  43. Jimmy

    Blackbody Field Conditioner (Audiophile Gear)

    http://www.lessloss.com/blackbody-p-200.html Only $1,323. Of course, they recommend multiple units to "maximize coverage and effectiveness." There is a lot of sciency stuff on their site and they have lots of other goodies to sell as well.
  44. C

    Blackbody Radiation Test Question

    Homework Statement The blackbody radiation equations of state are E/V=aT^(4) and P=(1/3)(E/V). From the fundamental state derive a quantity involving only P and V that remains constant when this radiation undergoes a reversible adiabatic transformation. Homework Equations Maybe...
  45. B

    Blackbody emissivity different to thermometers

    I am doing some research on thermometers and have a blackbody to test some infrared ones. Unfortunately the blackbody has an emissivity of 0.97 +/- 0.02 and the thermometer has an emissivity of approximately 0.96? How do I correct for the differences? I have had a quick look online and other...
  46. L

    Quantization of energy in blackbody radiation

    I have been reading a lot of stuff on blackbody radiation and the ultraviolet catastrophe. Here is what I have so far. The ultraviolet catastrophe arises from the classical electrodynamics predicting an infinite amount of energy from a blackbody having any temperature. As far as I have...
  47. sweetdreams12

    Blackbody Heat Shields/Metal Sheets

    A “blackbody heat shield” is used to reduce radiant heat loss in situations where standard low conductivity insulation (i.e. normal insulation) cannot be used. The shield is constructed using a set of parallel metal sheets, each with emissivity 1.0. Explain how/why this “heat shield” works...
  48. S

    Blackbody radiation derivative exercise

    Homework Statement The problem (18.1a) can be found here: http://www.suagm.edu/umet/paginas/dbacelo/chem464/scan-probcap18-levine-pag1.pdf . For reference, the equation (referred to as 18.2 in the problem statement) for the frequency distribution of blackbody radiation is give as...
  49. X

    Deriving the Total Energy from Blackbody Radiation: A Mathematical Approach

    This might be more of a mathematical question than a physical one. But I am taking a Quantum Mechanics course and the book starts out by introducing the equation for the energy density of radiation from a black body. They then integrate this expression over infinity to find the total energy...
  50. N

    Calculate the rate of energy output of the sun, taking the sun as a blackbody

    [b]1. The surface temperature of the sun is approximately 5800K. Given that the radius of the sun is 6.96x108m calculate the rate of energy output of the sun, taking the sun as a blackbody. [b]2. H = εσAT4 [b]3. Surface temp = 5800K, Radius of the sun = 6.96x108, Emissivity of a...
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