What is Blackbody: Definition and 193 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. B

    Blackbody radiation of beam of light

    Homework Statement a 100W beam of light is shone onto a blackbody of mass 2e-3 kg for 10e4 seconds. The blackbody is intially at rest in a frictionless space a) Compute the total energy and momentum absorbed by the blackbody from the light beam b) calculate the blackbody's velocity at the...
  2. M

    Planck's blackbody radiation law is proved untrue at nanoscale distances

    A well-established physical law describes the transfer of heat between two objects, but some physicists have long predicted that the law should break down when the objects are very close together. Scientists had never been able to confirm, or measure, this breakdown in practice. For the first...
  3. M

    How does a blackbody absorb radiation? and why?

    I've read that the phenomenon of black-body radiation was one of the key factors in the creation of quantum mechanics. So I want to learn about it...;-) First of all I don't understand why the the temperature of the black body increases when it absorbs energy from photons. Why can't the...
  4. D

    Blackbody Radiation Explained: A Layman's Guide

    Hi. I know this is a pretty basic principle, however I'm fairly new to the subject and was wondering if anyone is able to give a brief 'layman' explanation of why, as Planck's law states, at lower wavelengths the blackbody radiation falls to zero rather than continuing to climb as stated in the...
  5. A

    Radiation question from a blackbody

    Homework Statement What should be the average temperature on Pluto, at its 5.9 * 10^12 distance from the Sun? Treat Pluto as a blackbody. Assume that the effective solar intensity is 68 %. Homework Equations Power of radiation = (sigma)eAT^4 The Attempt at a Solution From the...
  6. P

    Integration problem to calculate partition function of a gase in a blackbody

    Homework Statement This is the integration i have to solve I=\int x^{2}In(1-exp(-ax))dx integration is from zero to infinity The Attempt at a Solution I know that it should be solved with integration by parts so u=In(1-exp(-ax)) du=[a exp(-ax)] / [1-exp(-ax)] dv=x^{2}dx...
  7. H

    Two Basic Blackbody Radiation quesions

    http://hyperphysics.phy-astr.gsu.edu/hbase/mod6.html#c4 This link shows the general plot of energy emitted by a blackbody against frequency. My two questions regarding this plot are: 1. Is the frequency in the plot the frequency of EM radiation emitted by the blackbody? 2. Why does the...
  8. H

    Blackbody in photoelectric effect

    any electron will be emitted if i use a black body as a metal target in photoelectric effect experimant and why??
  9. W

    Cannot Concentrate Blackbody Radiation

    Hi, having a bit of trouble with a basic question on thermal radiation. If you take a slab of material that has the characteristics of a blackbody and heat it to temperature where its only emitting in the IR, then its temperature will slowly decrease as the heat is rejected (assuming no...
  10. R

    Problem from Eisberg: Blackbody Radiation

    Homework Statement In case of Cavity Radiation(Blackbody radiation) let \rho_{T}(\nu)d\nu denote the energy density of radiation having frequency in the interval \nu and \nu + d\nu. Then we need to show that \frac{\int_{0}^{\lambda_{max}} \rho_{T}(\nu)d\nu}{\int_{0}^{\infty}...
  11. R

    Planck's formula for Blackbody Radiation

    Homework Statement Suppose that a blackbody spectrum is specified by Spectral Radiancy R_{T} (\nu) d\nu and Energy Density \rho_{T} (\nu) d\nu then show that R_{T} (\nu) d\nu = \frac {c}{4}\cdot \rho_{T} (\nu) d\nu Homework Equations \rho_{T} (\nu) d\nu = \frac{8 \pi h \nu^{3}...
  12. tony873004

    Help with Blackbody: Doubling \lambda_{max} with Temp Increase

    At a given temperature, \lambda_{max} for a blackbody cavity = 6500 angstroms. What will \lambda_{max} be if the temperature of the cavity walls is increased so that the rate of emission of spectral radiation is doubled? R_T = \sigma T^4 \,\,\,\, \Rightarrow \,\,\,\,T^4 = \frac{{R_T...
  13. C

    Does a Blackbody at 200K Emit Twice as Much Radiation?

    Homework Statement Does a blackbody at 200K emit twice as much total radiation as when its temperature is 100K? Homework Equations N/A The Attempt at a Solution I'm not sure, but I think it is yes because if the temperature is doubled, then the energy of the photons will be...
  14. T

    Heat Transfer, Blackbody Radiation

    Homework Statement The solar constant is the amount of energy from the Sun we receive on the Earth during each second on a 1.000 m2 area oriented perpendicular to the direction of the sunlight. The value of the solar constant is about 1.37 kW/m2. Imagine sunlight illuminating an asphalt...
  15. F

    Blackbody Radiation Problem

    Homework Statement The energy reaching Earth from the Sun at the top of the atmosphere is 1.36x10^3 W/m^2, called the solar constant. Assuming that Earth radiates like a blackbody at uniform temperature, what do you conclude is the equilibrium temperature of Earth? Homework Equations...
  16. N

    3K universal blackbody temperature radiation

    Homework Statement What does the 3K universal blackbody temperature radiation tell us about the temperature of the outer space? Homework Equations The Attempt at a Solution the 3K universal blackbody radiation is regarded as the remnant of the Big Bang.And this temperature looms all...
  17. B

    Blackbody radiation frequency problem

    Homework Statement The Cosmic microwave background radiation fits the Planck equations for a blackbody at 2.7 K. a) What is the wavelength at the maximum intensity of the spectrum of the background radiation ? b) What is the frequency of the radiation at the maximum? c) what is the total...
  18. A

    About the radiation of blackbody

    I am reading the book about the light resource, and I can not understand that why hot gas radiates E-M waves of discrete frequency, but hot solid radiates continuous waves. As you know, high intensity gas discharging lamps give out discrete spectrums, but tungsten resistance lamps give out...
  19. D

    Photons emitted by a blackbody

    Homework Statement Consider a 75 Watt light bulb and an 850 Watt microwave. If the wavelengths of the radiation they emit are 500nm and 150nm, respectively, estimate teh number of photons they emit per second. Are teh quantum effects important in them?Homework Equations ? M(T) = \sigma T^4...
  20. P

    Blackbody fraction of Radiation

    Homework Statement Determine the fraction of the energy radiated by the sun in the visible region of the spectrum (350 nm to 700 nm). (Assume the sun's surface temperature is 5800 K.) Homework Equations R=\sigmaT^{4} for some reason i can't make the sigma come down, but it's a...
  21. U

    Blackbody Radiation (Rayleigh-Jean) Equipartion of Energy Question

    There's one thing that has always puzzled me about the derivation of the old classical Rayleigh-Jean Law of blackbody radiation. I understand how they calculate the density of modes in the cavity however I don't see why they assign an "equipartition" energy of kT per mode instead of 1/2 kT as is...
  22. J

    The formation of blackbody radiation

    I am puzzled about the formation of blackbody emission (Planck's law). Specifically, we know that such things like incandescent lamps, an electric arc in a gas at high pressure etc. produce a nearly blackbody spectrum of corresponding temperature. Does this mean that in these cases a nearly...
  23. M

    Blackbody Radiation: Plank's Quantum States & Frequencies

    Hi, I'm just wondering if Plank suggested that the quantum states and frequencies emitted are quantized, then why do we see a "continuous" spectrum of the BB radiation, not just discrete bands?
  24. B

    Blackbody Radiation: Explaining Integer Frequencies

    I guess this is supposed to be here... I dunno. Anyhoo, I understand this concept intuitively, but why do frequencies have to have integer values? Is that true of all waves, or just electromagnetic waves? Could a wave have 33.43 Hz, for example? :cry: Please explain!
  25. U

    Blackbody Radiation and the Incandescent Light Globe

    Hi. I am wondering how well the filament in an incandescent light globe follows the basic shape of the classic blackbody temperature/spectrum curves. The reason I ask is that I just set a small science project for a student to measure the voltage and current of a small torch globe (4.8V 0.75A...
  26. P

    Derivation of Plank's Equation on Blackbody Radiation: Standing Waves Explained

    In the derivation of plank's equation on blackbody radiation, the radiation within the cavity must form standing waves and thus each frequency has a certain number of possible modes. However, I have no idea why the radiation has to form standing waves. Why does the electric field have to be...
  27. S

    Blackbody radiation - stat mech

    we have a hollow cubical box with sides of length a with perfectly conducting walls, such that the electric field tangential to the surfaces of the walls must be zero. we need to show that the system of standing waves: Ex = Ax*cos(Kx*x)*sin(Ky*y)*sin(Kz*z)*exp(iwt) Ey =...
  28. U

    Blackbody radiation formula

    I am to show Planck's blackbody radiation formula without the stimulated emission: level n is the upper energy level, level m is the lower energy level: N_m B u(\lambda,T)=N_n A \frac{N_n}{N_m}=\frac{B u(\lambda,T)}{A} N_n=ce^{-E_n /kT} N_m=ce^{-E_m /kT} \frac{N_n}{N_m}=e^{-h...
  29. S

    Managing Blackbody Radiation in Magnetic Confinement Fusion

    Even by taking low numbers for the surface area and emissivity of the plasma, the radiation would be extremely high due to high temepreatures in magnetic confinement fusion. So how is blackbody radiation countered in magnetic confinement?
  30. S

    Solving the Blackbody Problem for Main Sequence Stars

    Hi, I am having trouble with the last question in my assignment. It's on blackbody radiation and total power per area and whatnot. those questions i was able to do fine, however when i came to this problem i was a little lost.. here it goes, i'll type the whole thing out.. "The most...
  31. L

    Entropy of blackbody radiation

    Can someone point out any information (derivation or where it comes from) about this equation: S = v\int_0^\infty\phi(\rho,\nu)d\nu \nu is frequency of radiation. \rho(\nu) is radiation energy, v is volume. I'm getting this from Einstein's photoelectric effect paper. Thanks a lot!
  32. M

    Artificial Blackbody & Artificial Black Hole: Reality or Theory?

    artificial black hole hi on web i read smt about artificial black holes , but unfortunately it was not detailed. what i want to ask is: >is creating art. bh only a theory or did someone it really?
  33. L

    Blackbody radiation and stefans constant

    What is the rate of energy radiation per unit area of a blackbody at a temperature of 260K ? Where's the tricky part in this question? Surely the answer to this question can't be just stefans constant * T^4 ie. 5.67x10^-8 * 260^4 = 259.105 W/m^2
  34. tony873004

    Blackbody Radiation: Temperature of Accreted Gas

    This is actually a homework question, but its probably better suited for this forum. Gas accreting onto burned-out stars like white dwarfs, neutron stars, and black holes can often be seen in the X-ray part of the EM spectrum, at energies of about 10 keV. What temperature does this suggest...
  35. C

    Calculating Spectral Exitance for Planck's Law & Blackbody

    Hello, My question is in regards to Planck's Law and a blackbody: For the single lambda case I can readily find the spectral exitance. Alternately, if I substitute to create an integral in the form of x^3 / (e^x - 1) and integrate over all lambda, I reach Stefan-Boltzmann. No problems...
  36. N

    How Does Planck's Law Simplify for High Frequencies?

    Can someone help me with this please? B(f) = [(2hf^3)/c^2]{1/[exp (hf/kT) - 1]} - Planck law in terms of frequency. "Derive expressions for B(f) for the cases i. hf << kT, ii. hf >> kT". I've done the first bit, that's just using a Taylor expansion. The second bit is where I'm stuck. In...
  37. U

    Blackbody Radiation: An Overview

    Sorry if my physics isn't so good. I only go to High school. Anywayz here is what i understand about blackbody radiation. A blackbody is an object that completely absorbs all the radiation of any wavelength. right. Then blackbody radiation means that a blackbody emitting radiation. Here is what...
  38. G

    Derive a formula for blackbody radiation

    when we derive a formula for blackbody radiation, we say that each electromagnetic mode has energy nhf, n is an integer, h is Planck's constant f is the frequency of the radiation. We interpret n as the number of photons per mode. However, a deeper QM analysis involves replacing the...
  39. Nebula

    A couple Blackbody Problems?

    A couple Blackbody Problems? I'm a little confused about these two problems involving blackbodies, hopefully someone could give me a bit of insight. Thanks in advance. 1. For a blackbody we there is a frequency peak and a wavelennght peak. Let's call em v and w respectively. Now consider the...
  40. M

    Questioning EM Waves, Synchrotron Emission & Blackbody Radiation

    I'm reading an astronomy textbook, and I'm not sure about some things. The textbook says the EM waves are caused by accelerating charges. I don't understand how this is. I'm not sure, but my memory and intuition tells me that only charges moving in a sinuosidal (can never spell it) manner...
  41. W

    A question about the sun and blackbody

    Why is the spectrum of sunligt similar to that of a blackbody?
  42. Hydr0matic

    Blackbody Math Help: Calculating from Maxwell & Boltzmann

    http://www.phys.virginia.edu/classes/252/black_body_radiation.html Have a look at this site, specifically the calculation of Planck's formula. I'm sure you've seen it many times before. What I need help with is calculating a similar formula based on my own idea instead of the oven model...
  43. A

    The Universe's Blackbody Spectrum: Why is it Troublesome?

    Why is it bothersome that the universe has a blackbody spectrum?
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