What is Black body: Definition and 159 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. M

    Black Body Radiation: Does it Occur in a Vacuum?

    Consider a hollow cuboid with a small hole on one of it's sides(interior surface is mat black).and let this cuboid be in a vacuum.if we allow a light ray to pass trough this small small hole. from which surface would the re radiation occur? Does this happen? the amount of radiations...
  2. L

    Rate of emission from a black body

    What is the proof that the total rate at which radiation is emitted by a black-body radiator is proportional to the fourth power of the temperature? Does this result come from Planck's radiation law? If yes, how?
  3. F

    Black body radiation verses spectral lines.

    When objects are heated they produce blackbody radiation which varies with their temperature, and when the electrons in the orbitals of atoms are given energy they produce radiation at specific frequencies known as spectral lines. My question is this. When an object, say a piece of mental is...
  4. B

    Quantum - black body radiation

    Homework Statement Suppose you are inside a black body radiation cavity which is at temperature T. Your job is to measure the radiation field in the frequency interval from 1014 to 89 x1014 Hz. You have a detector to do the job. What should the temperature of the detector (T’) be? Explain...
  5. E

    How Do You Convert d3p to a Function of Frequency Nu in Spherical Coordinates?

    Homework Statement Write the volume element of d3p as a function of "nu". Assume spherical symmetry in doing this change of variables so write d3p = 4\pip2dp. Homework Equations n(\nu)=\frac{1}{e^{\frac{h \nu}{kT}} -1} \epsilon=\frac{2}{h^3}\int h \nu \cdot n(\nu)d^3p The Attempt...
  6. S

    Exploring Black Body Assumptions for Deriving Spectrums

    To derive the blackbody spectrum, we set up the situation by assuming that a cavity is in thermodynamic equilibrium with a heat bath. My questions are: 1) Is the heat bath the black body? 2) Why is it called a blackbody? 3) Why does the cavity have to be in thermal equilibrium with...
  7. C

    Black Body Radiation - graphical trend

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  8. N

    Black body radiation experiment

    Homework Statement im at the point where i ve to plot graphs of intensity versus wavelenght of a tungsten lamp for different temperatures.i ve calculated corectly the temperature and the wavelenghts..after that i took Planck s formula to find the corresponding intensity for each wavelenght and...
  9. L

    Looking for a picture of black body radiation from an oven

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  10. S

    Quantization to solve black body radiation

    Homework Statement how does Planck's idea of quantization of the energy found in electromagnetic waves solve the problem of black body radiation? Homework Equations N/A The Attempt at a Solution this is what i have said correct me if I am wrong... so plank said that photon has...
  11. fluidistic

    Black body radiation, question arising from a book

    In the book "Introduction to the structure of matter" by Brehm and Mullin, page 78. They say "It should be empathized that there is no reason for the peak positions \mu _m and \lambda _m in the respective distributions to be connected by the relation c=\mu \lambda. They are talking about the...
  12. B

    Black Body Radiation: Info for Non-Quantum Physicists

    Where can I find a lot of information on it that wasn't meant for a Quantum Physicist? I am pretty sure that once I understand that learning about EMR will be easier.
  13. fluidistic

    Can a Black Body Emit Only One Photon After Being Hit By a Single Photon?

    To make my question simpler, assume that there's a black body at 0K or very close to it (if we can't assume there's a body at 0K). Say I have a monochromatic source of photons which sends 1 photon on the black body. The wavelength of the photon is 500 nm so around green. If I understood...
  14. M

    Black body radiation entropy question

    Homework Statement An evacuated container with volume V and at a temperature T contains black body radiation with an energy density equal to 4\sigmaT4/c I Determine the heat capacity at constant volume of the radiation II Hence show that the entropy of the radiation is given by...
  15. C

    How Planck explained black body radiation

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  16. C

    Black Body Radiation (Awkward integral)

    Homework Statement What percentage of the Sun’s blackbody radiation spectrum falls into the visible light spectrum (400-700 nm). Where T=5000K Hint: Integrate over frequencies Homework Equations B=2h\nu3c-2 (eh\nu/kT-1)-1 Where \nu is the frequency of the light. The Attempt at a...
  17. M

    Can the Sun be treated as a black body radiator?

    Hello, I am trying to revise for my Solar System exam and going through a past paper i have a question relating to something that we don't seem to have covered: "By estimating the energy output of the Sun's corona (in watts), comment on whether can be treated as a blackbody radiator...
  18. S

    A black body covered by another black body.

    Question: A spherical black body is kept at constant temperature thermal equilibrium T and by supplying heat from some source. It is covered by another spherical surface which is also a black body (a very thin surface of radius almost equal to the previous one). Find the temperature at which...
  19. S

    Box Normalizing in Quantum Mechanics: The Role of Black Body Radiation

    What is box normalizing and why is it important in quantum mechanical problems such as the black body radiation?
  20. M

    Black Body Net Heat Absorption Problem

    Homework Statement A naked person, whose skin area is 1.7 m2, sits in a sauna that has a wall temperature of 61oC. If the person’s skin temperature is 37oC, find the net rate at which the person absorbs heat by radiative transfer (assume an emissivity e of 1). How much liquid must the...
  21. T

    Planck black body formula question

    Hi all -- I had a question about the Planck black body / Johnson-Nyquist resistor temperature formula: \[ E_{\nu}^{2}d\nu=\frac{4R_{\nu}hd\nu}{e^{h\nu/kT}-1}\] I'm just wondering why the formula has $ exp(h\nu/kT)-1$ in the denominator? I would have expected $ exp(h\nu/kT)+1$ as...
  22. I

    Black Body Radiation: Derivation & Justification

    Hey In several books, the derivation of black body radiation is done by considering the energy density inside a cavity surrounded by walls in a certain temperature. The derivation is described as one of the first steps in quantum mechanics, where all considerations are purely classical but...
  23. Z

    Do all bodies emmit black body radiation?

    Is this black body radiation considered light? PS: Sorry, I spelt emit wrong:(
  24. X

    White noise vs Black body radiation

    Hi all: I am confused about one question. When we detect signal from human brain using coil in Nuclear Magnetic Resonance, two source contribute noise to our signal. One is coil itself. The other is human brain. Why can we think the noise from human brain is white noise? Can we compare the...
  25. K

    Black Body in Motion Books: Find & Learn Relativity

    Hello! I am looking for books about black body in motion but i can't find any in Google. So, does anyone know something about it and let me know? Just even the address of where i can find it is enough. Actually, the black body that i know is the one which absorbs all incident light...
  26. N

    Black body radiation: can you help me understand it (better)?

    My apologies if I am posting this in the wrong forum, and if this question has already been asked. This is my first post. First off: I'm a first-year undergraduate student. My program is Philosophy of Physics. I am trying to understand (more fully) black body radiation. I am having trouble...
  27. S

    Proving I=(1/4)cE: Exploring Black Body Radiation

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  28. A

    Is There a Way to Convert Mass into Charge?

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  29. S

    Is a black hole considred a black body?

    Is a black hole considered a black body?
  30. L

    Simple Black Body Radiation Question

    Homework Statement The total radiation emitted by a certain star is a factor 16 times that of a second star of equal size. Calculate the ratio between the temperatures of the star. Homework Equations lamda x T = 2.90x10^-3 The Attempt at a Solution 16I = I Therefore, Wiens Displacement law...
  31. C

    Calculate number of modes incident on detect from black body

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  32. Z

    Understanding Plank's Law of Black Body Radiation

    Plank’s Law which descibes black body radiation is derived from first principles so I am not quite sure why it is called a Law. http://en.wikipedia.org/wiki/Plank_law"
  33. P

    What is the Black Body Radiation Integral?

    Find\int\frac{x^3}{e^x-1} evaluated between zero and infinitum. I got...
  34. J

    Black Body Radiation Homework: Energy/Sec on 1m2 Earth from Sun

    Homework Statement Assume that the radiation emitted from the Sun moves radially outward from the Sun and that no radiation is absorbed between the Sun and Earth. How much energy in the form of radiation will fall per second on an area of 1 m2 on Earth, if that area is perpendicular to the...
  35. S

    Black Body Radiation: Find Temperature of Panel PM in Radiation Equilibrium

    ***** SHEETS two ideally black of the area 1:5 mETER SQUARE are located opposite of each other in vacuum. The first SHEET P1 has the FIXED temperature T1 = 0o C, the second SHEET P2 has the fIXED temperature T2 = 100o C. Now an additional black panel PM of the same size is brought between the...
  36. fluidistic

    Why does a black body never turn green when heated?

    I think it has been asked a lot of times and explained a lot of times, but as I've never seen the answer I post my question here. Why does a black body never becomes green when it is heated? It pass from infrarred to red, orange, yellow but instead of passing to green, it then passes by...
  37. P

    2nd law of Thermodynamics and black body brightness function

    Hi, I feel extremely stupid for having to have this explained to me, but I am really confused by it. I recently encounter a proof of the fact that the brightness (energy/(time x area x solid angle x frequency)) of a black body can only depend on its temperature, not the properties of the...
  38. H

    Exploring Black Body Energy Spectrums: hv vs kbT

    The question is as follows: The black body energy spectrum is \rho(T,v)dv=\frac{8\piv2<E(v)>}{c3}dv where v is the frequency of the EM wave and <E(v)> is the average energy at v. Assuming the energy of a EM wave of v can only take multiples of hv, the from the Boltzman probability...
  39. Amith2006

    Black Body Radiation: Why Maintaining Constant Temp?

    # Hi guys! One question is troubling me a lot. It is know fact that a hollow metallic enclosure with a small hole is a perfect black body. When electromagnetic waves of any frequency are incident on the hole, it is completely absorbed after a number of reflections. So, the condition for...
  40. B

    Explaining the Color Change of Black Body Radiation from Coal

    taking a lump of coal as an approximation of a black body, what is the explanation for the phenomenon of the emitted light changing colour from red to white when it is heated, ie the emitted quanta themselves changing, rather than simply more being released.
  41. H

    Black Body / Emission difference ?

    If I make a ball out of calcium; drill a little hole in it; then heat it up in a suitable [vacuum] environment; I see black body radiation thru the little hole, right? At least up to the melting point of calcium ?? I mean, black body radiation is independent of material, right ? Yet the...
  42. Andre

    Earth black body temperature wrong?

    The blackbody temperature of Earth is supposed to be -18C or 255K. This can be derived by reworking Stefan Boltzmann law to: TK = (S*(1-a)/4*rho) ^0,25 http://www.lwr.kth.se/Grundutbildning/1B1292/Compendium_online/ch05s02s01.html in which S is solar influx, we use 1367 w/m2 a is...
  43. J

    How Does a Flat Black Body Radiate Energy into a Hemisphere?

    Im working on designing a non contact thermal instrument. Need to estimate how much current I'll get from the photodiode. First background: I'm working from Planck's law and Stephan's law. The temperature of my target will be between 1250 and 1750K. From Stephan's law I've worked out this...
  44. A

    Density of black body radiation

    I came across a formula that gives radiant energy density as 4 * sigma * T^4 / c. I am trying to figure out how this formula follows from Stefan-Boltzmann law. Shouldn't it be 4*pi instead of just 4? I am also wondering why one can just stick in c. TIA
  45. L

    Black Body Radiation Calculation: Unveiling My Cluelessness

    I was looking over the calculation leading to the thermal average number of photons s in a mode of frequency w in a black body. The approach was pretty straightfoward: Calculate the partition function Z based on quantized energies of a harmonic oscillator, then use this to calculate: <s> \ = \...
  46. M

    Ultraviolet Catastrophe / Rayleigh-Jeans Black Body Cavity

    In reading about the “Ultraviolet Catastrophe” in dealing with black body radiation, my book says that at the ultraviolet frequencies and beyond, the Rayleigh-Jean equation for the energy density of the radiation would be infinite (and thus a catastrophe). If this is the Rayleign-Jeans...
  47. Amith2006

    How do I find the work function of a metal using black body radiation?

    Homework Statement 1)The peak emission from a black body at a certain temperature occurs at a wavelength of 9000 Angstrom. On increasing its temperature the total radiation emitted is increased 81 times. At the initial temperature, when the peak radiation from the black body is incident on a...
  48. T

    Differentiating Black Body Energy Density: Seeking Answers

    Hi all, I hope everyone is well and that life is treating you all good. I am going to be honest with my question and say that I have not tried to do it myself yet. I, unfortunately, do not have time now to try and then repost so I do hope you will all forgive me for just stating a...
  49. Amith2006

    Black Body Radiation: Why Does the Black Spot Appear Brighter?

    Sir, A polished metal plate with a rough black spot on it is heated to about 1400 Kelvin and quickly taken into a dark room. It is said that the black spot will appear brighter than the plate. Is it because the black spot is a perfect black body and hence is a better absorber of heat than...
  50. H

    How can you calculate the power intercepted by a planet from a distant star?

    Hi, I found one very interesting physics problem but I have no idea how to solve it. Lets say we have a star with radius R and the surface temperature T. Now we wish to know, what is the ratio between the total power output of the star and the power that the distant planet receives from...
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