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. P

    What Does Black Light Do to a Blackbody?

    if a wave of light intermingles with another the same but of opposite phrase I believe you get what is called black light. If the black light hits a blackbody what happens? does any radiation energy heat up the black body?
  2. 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...
  3. 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...
  4. R

    Understanding Planck's Blackbody Law: A Comprehensive Guide

    I don't see how this follows.
  5. 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...
  6. R

    How is the Entropy of Blackbody Radiation Calculated?

    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...
  7. G

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

    Homework Statement A blackbody is radiating at a temperature of 2.50 x 103 K. a) What is the total energy density of the radiation? b) What fraction of the energy is emitted in the interval between 1.00 and 1.05 eV? c) What fraction is emitted between 10.00 and 10.05 eV? Homework...
  8. H

    Blackbody Radiation Homework: Understanding the c/4 Factor

    Homework Statement http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/radpow.html#c1 What I don't understand is the second part, with the angles. The more I think about it, the less it makes sense. Homework Equations R = c/4 U The Attempt at a Solution Whenever I did a surface integral in...
  9. R

    IR thermometers and Blackbody Radiation

    My question is about using an IR thermometer for measuring the temprature of a blackbody that emitts a spectrum that peaks somewhere in the visible region. I think since these kind of thermometers measures the amount of infrared radiation emitted by the object, they can only determine the...
  10. P

    Is the sun a passive or active blackbody?

    Is the sun a passive or active blackbody? Thanks
  11. R

    Cosmic microwave background fits the blackbody radiation spectrum

    Homework Statement (a)The cosmic microwave background fits the blackbody radiation spectrum well with a temperature of 2.7 K and a corresponding peak wavelength at 1.9nm. Applying the relationship between the radiant emittance, i.e. the total power emitted per unit area, and the photon energy...
  12. 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...
  13. T

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

    Homework Statement Imagine you are trying to detect a Jupiter-like planet located 7.5E11 m from a Sun-like star. At what wavelength is the peak of the planet's Planck blackbody intensity function (assume the planet is a perfect blackbody)? Radius of Planet r = 7E7 m Radius of Star R =...
  14. T

    How to Calculate the Radius of Sirius A and B?

    Homework Statement We are asked to calculate the flux at a specific wavelength (f_5500), the surface flux(F) and the radius of a star (Sirius A and then B) We are given the following: Brightness of Sirius A: V=-1.47 Effective temperature: 9870K Central wavelength: 5510 Angstroms Filter...
  15. S

    Theory question - Blackbody Radiation and Light

    "Theory" question -- Blackbody Radiation and Light I am trying to understand the discussion about blackbody radiation in my Modern Physics textbook. (I'll quote it, but it can be found http://phy240.ahepl.org/Chp3-QT-of-Light-Serway.pdf" , page 5 document numbering, 69 textbook numbering)...
  16. N

    Blackbody radiation with frequency filter

    Let's say there's a perfect blackbody inside a box (at room temperature) composed of material such that only x-rays are able to enter and leave. If the absorption and emission of the x-rays weren't the same, the box would heat up above room temperature, and this would contradict thermodynamics...
  17. N

    Equating Units in Plank's Blackbody Equation

    Homework Statement My question is, why, when changing the variable from "v" to "lambda", does the d\lambda need to be included? Why can't a simple substitution c = v * lambda do the trick?
  18. cepheid

    Earth Blackbody Temp: How Albedo Affects Result

    I saw a claim today that without the greenhouse effect, Earth's surface would be on average at a chilling -18°C (note: this is not a climate change thread). I set about trying to reproduce this result, so at first I assumed that Earth was a perfect blackbody, and that in order to be in...
  19. P

    What causes blackbody radiation

    I've seen the cavity resonator model but no one has been able to explain to me what the underlying physical mechanism is that generates the blackbody radiation continuum. Is there some intuitive explanation or is it deeply rooted in quantum mechanics? An explanation I've heard is that it's...
  20. S

    A few questions on blackbody radiation

    Homework Statement Suppose we have a copper foil at room temperature 300 K°, in the dark. If we irradiate it on both sides with laser beams at frequency , say 10^ 13 Hz, one would expect a temp of 500 K° and EMR at 10^ 13. Actually it radiates , according Plancks law, at higher...
  21. K

    Why does blackbody radiation exist?

    It seems it is predicted by both classical and quantum physics. Why is it predicted in the first place? Is it purely a mathematical reasoning, or is there some qualitative reason why it must exist? In other words, why would some philosopher/physicist hundreds of years ago say to himself, "I...
  22. P

    Blackbody radiation and Planck's distribution

    physics books on the topic of blackbody radiation say that any given object when heated will emit a continuous electromagnetic radiation which depends only on temperature. Planck's distribution based on discretization of energy states accurately describes this phenomenon. But is this true? If...
  23. S

    Concentrating radiation from blackbody with lens

    1)Is it possible to concentrate infrared radiation from a blackbody with a lens? 2)Should a lens be at different temperature from a blackbody?
  24. B

    Blackbody Radiation: Why Bodies Absorb & Radiate Heat

    If I may dare to ask the question: WHY are bodies absorbing and radiating heat in same proportion? I can't find the answer anywhere on the internet. And why are bodies radiating heat in the first place? And why whole spectrum of wavelengths?
  25. S

    Blackbody Radiation of Boiling Water

    I was thinking about this the other day and I wanted some other input on the matter. Cups of boiling water (I was thinking about coffee) give off gobs of IR radiation at both near and mid wavelengths. Could I think of a coffee mug as an approximate black body for IR radiation? The insulation of...
  26. S

    Blackbody Spectrum: 2 Questions Answered

    Hi all, I have 2 questions related to the blackbody spectrum. 1 - If a blackbody emits (and absorbs) energy at all wavelengths equally, why does the spectrum look the way it does (i.e. why does it peak in intensity at a given wavelength?) 2 - The fact that the cosmic background radiation...
  27. S

    What causes deviations between Sun and blackbody?

    Hello all, Does anybody have an idea why the solar spectrum, as measured above the Earth's atmosphere, has such bad agreement with the blackbody curve? I would think that any deviations such as sunspots would be averaged out because of the distance, and that in general (if any deviation)...
  28. B

    Blackbody effect hypothetical question

    According to the blackbody effect, can something be so hot that it emits no visible light? I say this because the more heated an object is, the more it moves away from the visible light spectrum, does that apply until there is no light at all?
  29. C

    Blackbody Radiation: 100K to 1000K Energy Increase

    Homework Statement By what factor does the radiated energy increase when a blackbody changes temperatures from 100K to 1000K a) 100 b) 6600 c) 1 x 10^ 4 <--- marked as the correct answer on the key. d) 5.7 x 10^4 e) 1 x 10^6 Homework Equations 1) f(peak) = 5.88x10^10*T 2) E =...
  30. S

    Thermal Motion to Blackbody Radiation Derivation

    Good Day, I was wondering whether anyone has a reference or can point me in the right direction for a mathematical derivation of how the photons in blackbody radiation (for solids) are generated from thermal motion. Regards Max
  31. B

    Entropy Change in Blackbody Radiation System in Thermal Equilibrium

    An evacuated container with volume V and at a temperature T contains black body radiation with an energy density equal to 4T4/c S(T,V)=16VT3/3c The container is placed in thermal contact with a heat bath at temperature Tr. If the heat capacity of the cavity material itself is...
  32. A

    If light is quantized, why are EM spectrum and Blackbody spectrum continuous?

    If light is quantized, and is given out in packets, why are the EM wave spectrum and the black body spectrum continuous? I am very confused, can someone offer some explanation? Any input is greatly appreciated.
  33. P

    Can Blackbody Radiation Explain Heat Absorption in Hollow Cavities?

    I'm reading from an introductory text on quantum physics, and came across this sentence: It's the second sentence that I don't understand: how can the energy in the EM field be responsible for the ability of a hollow cavity to absorb heat?
  34. A

    How to Calculate Photon Flux from a Blackbody Source?

    Had a quantum assignment recently and one of the problems was: What is the photon flux (photon cm^-2 s^-1) at a wavelength of 600nm at a distance of 1km from a blackbody soiurce at 2000k emitting 50W of radiation between 400nm-700nm. My thinking was: This involves Planck's radiation...
  35. C

    Blackbody energy detector, from Liboff's QM

    Homework Statement Problem 2.11 of Liboff's Introductory Quantum Mechanics, 1st edition Suppose that you are inside a blackbody radiation cavity which is at temperature T. Your job is to measure the energy in the radiation field in the frequency interval 10^{14} to 89 \times 10^{14}...
  36. D

    What Is the Optimum Band Gap for a Solar Cell Based on Blackbody Radiation?

    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...
  37. W

    Blackbody Radiation and Emissivity Explained

    Hi All, I'm reading about blackbody radiation and emissivity and I came upon the following statement "Imagine looking into a small opening of a deep cave. In the visible wavelengths, the opening looks black because the light that enters the cave is not easily reflected back out. However...
  38. 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...
  39. 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...
  40. 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...
  41. 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...
  42. 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...
  43. 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...
  44. 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...
  45. 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?
  46. 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...
  47. 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
  48. 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...
  49. 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) =...
  50. 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...
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