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. D

    What is the name for a curve similar to a blackbody radiation curve?

    I was plotting some data and got a curve that was instantly recognizable as a blackbody radiation curve. The data has nothing to do with blackbody radiation though, and I was wondering if there was a more general name for this type of curve? To be clear, I mean a curve like these...
  2. T

    Blackbody radiation intensity find maximum

    Homework Statement By direct calculation, derive and expression for the wavelength λ(max) at which blackbody radiation intensity is a maximum [b]2. Homework Equations Planck's law for the intensity distribution of blackbody radiation as a function of wavelength λ and the blackbody...
  3. R

    Understanding Planck's Blackbody Law: A Comprehensive Guide

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  4. 4

    Blackbody Radiation: Solving Introduction to Cosmology Eq. #25

    Hello. I am trying to study "Introduction to Cosmology" by Barbara Ryden, but I am stuck with an equation from chapter two, and I have no idea how to figure out this. If you can suggest me a reading material or can explain the equation to me, that would be wonderful. It is page 20 of the...
  5. R

    Entropy of Blackbody Radiation

    Homework Statement The following is from a book: "Terrestrial radiation, T=255 K. Emitted flux ≈ 240 W m-2. Energy density for cavity radiation ≈ 3x10-6 J m-3. Entropy for cavity radiation ≈ 1.7x10-8 J K-1 m-3." I can't understand how they have calculated the Entropy.The Attempt at a...
  6. G

    What is the energy density of blackbody radiation at 2.50 x 10^3 K?

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  7. H

    Blackbody Radiation Homework: Understanding the c/4 Factor

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  8. R

    IR thermometers and Blackbody Radiation

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  9. P

    Is the sun a passive or active blackbody?

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  10. R

    Cosmic microwave background fits the blackbody radiation spectrum

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  11. D

    Number of photons in a mode (in a blackbody)

    # of photons in a mode (in a blackbody) I've been going over a proof again concerning the thermal average number of photons in a mode of a cubical blackbody of length 'L' at equilibrium with a reservoir of temperature 'tau' (has a small hole in it, etc. etc. etc.) The logic went as usual...
  12. T

    At What Wavelength is the Peak of the Planet's Blackbody Intensity Function?

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  13. T

    Calculating radius of a blackbody

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  14. S

    Theory question - Blackbody Radiation and Light

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  15. N

    Blackbody radiation with frequency filter

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  16. N

    Equating Units in Plank's Blackbody Equation

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  17. cepheid

    Earth Blackbody Temp: How Albedo Affects Result

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  18. P

    What causes blackbody radiation

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  19. S

    A few questions on blackbody radiation

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  20. K

    Why does blackbody radiation exist?

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  21. P

    Blackbody radiation and Planck's distribution

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  22. S

    Concentrating radiation from blackbody with lens

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  23. B

    Blackbody Radiation: Why Bodies Absorb & Radiate Heat

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  24. S

    Blackbody Radiation of Boiling Water

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  25. S

    Blackbody Spectrum: 2 Questions Answered

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  26. S

    What causes deviations between Sun and blackbody?

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  27. B

    Blackbody effect hypothetical question

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  28. C

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  29. S

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  30. B

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  31. A

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  32. P

    Can Blackbody Radiation Explain Heat Absorption in Hollow Cavities?

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  33. A

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  34. C

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  35. D

    Blackbody radiation spectrum

    Homework Statement I'm trying to calculate the optimum band gap for a solar cell for an essay I'm writing, but I am missing a step somewhere along the way. A solar cell has an energy transition E. Any photons with incident energy hv<E are not absorbed. Any photon with energy hv>=E are...
  36. W

    Blackbody Radiation and Emissivity Explained

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  37. B

    Blackbody Radiance: Calculate Absorption & Emission Rates & Equilibrium Temp

    Homework Statement Consider a slab of matter contained within two infinite planes a distance of s = 1m apart. absorption coefficent =1m2kg-1 density of slab 1kgm-3 heat capacity 103^3jk-1kg-1 assume the absorption coefficient to be independent of wave length 1.Suppose one face of...
  38. B

    Blackbody Radiance Homework: Calculating Warming & Cooling Rates

    Homework Statement Consider a slab of matter contained within two infinite planes a distance of s = 1m apart. absorption coefficent =1m2kg-1 density of slab 1kgm-3 heat capacity 103^3jk-1kg-1 assume the absorption coefficient to be independent of wave length 1.Suppose one face of a...
  39. B

    Question on blackbody radiance

    Hi, just looking for some pointers in how to solve the following question, such as formulas, etc, any help at all so as i can proceed in the right direction will be much appreciated, thanks in advance. not sure how to start it Consider a slab of matter contained within two infinite...
  40. S

    Calculating Flux, Blackbody Surface Fluxes, and Radii of Sirius A & B

    Homework Statement The central wavelength and the flux calibration are given in Table 2 (f5500= Cv * 10^-0.4V) Calculate the flux measured at Earth f5500, the blackbody surface fluxes B5500, and the radius (in m and solar radii) For Sirus A and B Table 1: brightness Sirius A: Va= -1.46...
  41. P

    Temperature of blackbody (star)

    Homework Statement b) Two stars, both of which behave like black bodies, radiate the same total energy per second. The cooler star has a surface temperature, T, and 3.0 times the diameter of the hotter star. i) What is the temperature of the hotter star in terms of T? ii) What is the ratio of...
  42. A

    Blackbody radiation - derive expression for T

    Homework Statement Derive an expression for the temperature of an ideal blackbody so that its radiated light at the peak intensity wavelength has exactly enough energy to excite the ground state to fourth excited state. Homework Equations I = σ*T4 λm*T = 2.9 * 10-3 m * K E = hf = hc/λ...
  43. L

    Blackbody radiation calculation problem

    Blackbody radiation problem! Homework Statement The average person has 1.4 m^2 of skin at a skin temperature of roughly 306 K. Consider the average person to be an ideal radiator standing in a room at a temperature of 293 K. a.) Calculate the energy per second radiated by the average person...
  44. J

    Average wavelength for blackbody radiation

    The maximum wavelength for blackboby radiation is .29/Temperature. Is the average wavelenght 1.84 times the maximum for all temperatures?
  45. M

    What causes the shape of the blackbody radiation graph?

    Hello, I got a question about this famous graph of the spectrum given by a heated body, showing the classical line (which goes up) and Planck's line (which gives the hill shape). How is this line continuous? Let's say my body is a lump of hydrogen, the only points on the graph should be the...
  46. TrickyDicky

    Blackbody Prediction for CMBR: Alpher, Herman, and Gamow's 1948 Theory

    I know the CMBR temperature was approximately predicted by Alpher ,Herman and Gamow around 1948, but I can't locate a source specifying when and how the blackbody form of the CMBR was predicted. Does anybody have this information? thanks
  47. O

    Tricky Derivation of Blackbody Equations.

    Homework Statement Starting with the Planck distribution: R(\lambda,T) = \frac{c}{4} \frac{8 \pi}{\lambda^4} (\frac{hc}{\lambda})(\frac{1}{e^{hc/(\lambda kT)}-1}) Derive the blackbody Stefan-Boltzmann law (ie total flux is proportional to T4) by integrating the...
  48. O

    Derive Stefan-Boltzmann Law from Planck Distribution for blackbody radiation

    Homework Statement Starting with the Planck distribution R(\lambda,T) for blackbody radiation. (a) Derive the blackbody Stefan-Boltzmann law (ie total flux is proportional to T4) by integrating the above expression over all wavelengths. Thus show that R(T) =...
  49. T

    Does Optic Focusing Contradict the Second Law of Thermodynamics?

    Ive heard a black body described as 'a body which absorbs all the radiation that falls upon it'. This seems to me to contradict the idea that a black body emits exactly as well as it absorbs light. I can understand that if you shine light matching a blackbody curve (at a given temperature) on...
  50. E

    Doppler shifted blackbody spectrum

    You can know the temperature of a star by fitting a black body spectrum. BUt what if the star is moving with some radial velocity v? I worked out that: I(\lambda_0,T)=\frac{8\pi h c}{\lambda_0^5}\frac{1}{e^{\frac{hc}{\lambda_0kT}}-1}...
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