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. PainterGuy

    I Blackbody radiation and the cosmic microwave background

    Hi, The following is my basic understanding of blackbody radiation spectrum. The important sections are in boldface. Source: https://en.wikipedia.org/wiki/Black-body_radiation#Spectrum Question 1: The quote above says that at room temperature (let's say 20 C or 293 K) the emission is in the...
  2. B

    Interpreting Weird Results: J_u, P_1 and T_1

    I need someone to check my work, because I'm getting weird results that I'm not able to interpret physically for parts b and c. Thanks in advance. For part a... ##J_u = e_1 \sigma_B T^4## ##P_1 = AJ_u = e_1 \sigma_B AT_1^4## ## T_1 = \left( \frac {P_1} {e_1 \sigma_B A} \right)^{\frac 1 4} ##...
  3. C

    I Assumptions for blackbody spectra vs. emission spectra vs. absorption spectra

    Hi there, I am a physical oceanographer teaching an introductory undergraduate Earth science class that has a unit on astronomy. I have a physics undergraduate background, took a few astronomy classes at the undergraduate level back in the day, and did a bit of undergraduate research in...
  4. M

    I Blackbody radiation in quantum mechanics

    Hello! If I place a particle with more energy levels (of the order of kT) in a well defined state, in a thermal bath at temperature T, how will the blackbody radiation affect the internal state of the particle i.e. will the distribution be classical or QM? Basically, if I prepare that particle...
  5. I

    I Relation between blackbody radiation and spontaneous emission

    I'm wondering what the relationship between blackbody radiation and spontaneous emission is. As far as I know, there are three sources of EM radiation - thermal radiation, oscillating dipole (multipole?), and LASER. And it seems like light emission from an atom can be separated into two...
  6. N

    I Blackbody colour of metals versus stars

    Hello. I am new to this forum and joined because I am at home nerding out trying to work something out. Why do white hot metals seem to be much cooler than white hot stars. The attached picture is from Wikipedia relating temperature of a hot metal to its temperature. For example a red giant...
  7. A

    Exploring the Relationship between CMB Radiation and Blackbody Temperature

    Here is something that struck a note to me, they give the CMB radiation in it's frequency which is in Ghz as the name "microwave" implies and then they also give a temperature in Kelvin. But how can light aka EM radiation have a temperature? I thought only matter with mass can have a temperature...
  8. tanaygupta2000

    Molar Specific heat of Blackbody radiation

    For a body at temperature T, the radiative energy per unit area E depends on 4th power of T. I can obtain expression for specific heat c by differentiating Stefan's law with respect to T. Would it be the correct way of approaching this problem? Or do I need to employ certain models from Solid...
  9. R

    I A qualitative question about Blackbody Radiation

    would this be a correct understanding of blackckbody radiation phenomena? in particular the intensity versus wavelength curve? "A Blackbody consists of oscillators of molecular dimensions. Intensity is proportional to number of oscillators with sufficient energy hv emitting radiation, and that...
  10. Viona

    I Blackbody Radiation: Questions Answered on Causes & Oscillators

    Hi every one I have three simple questions: what causes Blackbody radiation? what does Planck meant by the oscillators? why the Blackbody radiation is continuous while the emission spectra of atoms (gases) is discrete? I know that the discrete emission spectra of atoms was explained by Bohr's...
  11. K

    I Why is a resonant frequency assumed in blackbody radiation?

    Hello! In the descriptions of the BB radiation that I read, I see that we assume we have a cavity at a fixed temperature in equilibrium, we make a hole in it and we look at the intensity of different frequencies emitted through that hole. As far as I understand, the intensity dependence on the...
  12. A

    I How are apparently 'continuous' blackbody spectra formed?

    My understanding of this is based upon the assumption that energy level transition is the only mechanism responsible for blackbody photons.
  13. UMath1

    I How can a blackbody get hotter?

    If rate of emission = rate of absorption for a blackbody, how can a blackbody undergo temperature change through radiation? How come black cars get hotter on a sunny day?
  14. Charles Link

    A On the Planck Blackbody Function

    The graph of the Planck blackbody function has an interesting feature:## \\ ## ## \rho_o=\frac{\int\limits_{0}^{\lambda_{max}} L_{BB}(\lambda,T) \, d \lambda}{\int\limits_{0}^{+\infty} L_{BB}(\lambda, T) \, d \lambda} \approx .2500 ##, where ## \lambda_{max} ##, in an exact derivation of...
  15. JulioHC

    Derivation of blackbody radiation equations for stars

    Good evening, As part of my course, I had this week two lectures about the blackbody radiation and its relation to the stars. While I do understand how to use results such as the Stefan-Boltzmann law and Wien's Law I'm lost in other parts. I think the only parts that I don't understand yet are...
  16. K

    A Explain the demonstration of Kirchhoff's law on blackbody

    The Kirchhoff law of dependence of energy emitted from a blackbody only on wave length and temperature was demonstrated by Kirchhoff in a paper in the year 1859. The paper is in german naturally but I found it translated in english in a book of prof. Brace in 1901. A lot of time ago. But my...
  17. S

    Understanding Net Energy Transfer Rate in Parallel Plate Setup

    Homework Statement Two parallel plates plates are maintained at temperatures ##T_L## and ##T_R## respectively and have emissivities ##\epsilon_L## and ##\epsilon_R## respectively. Given the Stephan-Boltzmann constant ##\sigma##, express the net energy transfer rate per area from the left plate...
  18. F

    What is the Specific Heat of an Ideal Blackbody?

    Hello, I was thinking about how a blackbody (and any other type of body) eventually reaches a steady-state, constant and finite temperature once the absorbed energy is equal to the emitted energy. The specific heat of a substance indicates the temperature change causes by the...
  19. heavystray

    B Temperature based on colour index using Planck's law

    Hi, I'm trying to find temperature of stars using the stars' B-V magnitude by using the Planck law. However i do not know how to solve for T (assume other quantities are all given and determined first). Any idea how to do so? I already tried to do it but reach a dead end. Here I attached the...
  20. nmsurobert

    I Are Stars Absorbing and Re-Emitting Radiation from Space?

    Ive been reading about it for a while and I'm still confused. i understand that a star is not a perfect blackbody. but do stars absorb radiation from space and then re-emit that energy into space?
  21. Arup Biswas

    Blue Light from a Blackbody Radiator

    From Wien's law it is seen that approx 6500K(Greater than sun's temp) needed to heat a body to blue! But we see so much blue light(uv ray)! Are they formed from any different mechanism rather than heating?
  22. Arup Biswas

    Blackbody Radiation: Explained!

    When I study any book of Quantum Mechanics like Resnick or Beiser etc all start with blackbody radiation! But how this radiation is produced? Google says due to increased collision of particles causing the acceleration and em wave but what particles? How they are accelerated from what? Like if...
  23. T

    Why is blackbody radiation continuous?

    Plasmas can emit radiation based on the acceleration of charged particles (which we generally consider as continuous), but for un-ionized matter compounds, transitions are quantized and photons have particular energies. At room temperature, collisional excitations are typically dominant. But if...
  24. Ron19932017

    I How to approach a more perfect blackbody?

    I wonder how an object, like our sun, can approach a more perfect blackbody. We know that by the wiki definition, blackbody is something that absorb all radiation and is in thermal equilibrium. Its spectrum only depends on T. We also know that, our Sun's spectrum is blackbody like, while a...
  25. N

    Calculating Losses for a Blackbody Heater and Infrared Sensor Geometry

    Homework Statement At room temperature, an infrared sensor with an area 0.1mm2 is suspended 0.1m above a disc that is a blackbody heater of diameter 0.05m . The infrared detector absorbs all thermal infrared radiation (0.2 - 100 microns) produced by the blackbody heater. What are the losses...
  26. S

    B Blackbody Definition: Absorbs All Light & Emits Thermal Radiation

    Hello! I am not sure I understand the concept of blackbody. The definition is that it absorbs all the incoming light and doesn't reflect any. However, it emits thermal radiation. Isn't thermal radiation still EM radiation, so technically still light, just at another frequency? So if it emits...
  27. J

    B Blackbody Radiation: Explaining Ultraviolet Catastrophe & Planck's Solution

    Hello, I'm an English student and external candidate, hoping to take my Physics with me through life. I have some questions regarding a topic I'm researching, currently. I have a book "Advanced Physics - Steve Adams, Jonathan Allday", which details 'Blackbody Radiation', as evidence for a...
  28. henry wang

    How does the surface property affect blackbody radiation?

    In an attempt to explain why a matt surface of aluminium is a better emitter/absorber of blackbody radiation than shiny surface of aluminium, my university lecturer suggested to me that: By brushing a metal surface to create a matt finish, the surface of the metal becomes rougher. Rougher means...
  29. Samama Fahim

    Spectral Intensity as a Function of Wavelength in Blackbody Radiation

    A blackbody is also a perfect emitter giving off electromagnetic waves at all frequencies. A detector could measure the intensity of the radiation it receives through the prism. By moving the detector to different positions, you could measure the intensity of light as a function of color or...
  30. snoopies622

    Blackbody and gas molecule speed graphs look similar. Why?

    The graphs of intensity versus wavelength of blackbody radiation and of number of molecules versus speed in a gas look very similar to me. Is this just a coincidence? They seem like quite different phenomena.
  31. G

    I Discrete vs Continuous Spectra in Blackbody Radiation?

    I was reading this article which talks about the theoretical model behind blackbody spectra: http://www.cv.nrao.edu/course/astr534/BlackBodyRad.html At the start, it mentions standing waves in a cavity. Standing waves in this model consist of an integer number of wavelengths. The standing waves...
  32. Artlav

    I Relativistic doppler shifted blackbody color?

    I'm trying to render the sky as it would appear from a starship moving at some large part of the speed of light. Geometry was straightforward, but colors are the problem. How would the doppler effect change the color of a star? The expectation is that the stars behind are red and the ones ahead...
  33. T

    Blackbody Radiation - Peak wavelength

    Homework Statement The Planck blackbody spectrum is given by u(ω,t)=\frac{ħω^3}{π^2c^3(e^{βħω}-1)} Show that the peak of the Planck spectrum for a blackbody at a temperature T occurs at the wavelength λ_{max}T=0.29 where T is in Kelvin and λmax is in cm. Homework Equations...
  34. E

    How to calculate blackbody temperature from radiance band?

    I know how to calculate blackbody radiance in a spectral band from a temperature by integrating (or summing) planks function. How can I do the reverse? For example, I have a Radiance of 1000 W/m^2/sr in the 3-5um band, I'd like to be able to calculate 666.6K.
  35. Neha98

    Verify particle nature of electron using blackbody radiation

    It's mentioned in my book that blackbody radiation can verify the particle nature of electron could anyone explain this to me,please?
  36. M

    I Problem in Deriving Rayleigh-Jeans Formula

    Hi I am having trouble with the derivation of the Rayleigh-Jeans from the steps shown at the hyperphysics web site. I have emailed Dr. Rod Nave who is listed as the person-in-charge at the site but I have been able to get a reply. It would really be much appreciated if anyone can have a look at...
  37. S

    Blackbody paradox applied to interaction of gravity with electrons

    Homework Statement How does the blackbody paradox argument show that the electromagnetic field cannot be classical while electrons and atoms are quantum mechanical? Should the same arguments apply to treating gravity classically and electrons quantum mechanically? Homework Equations The...
  38. T

    I Distinguishing Blackbody and Atomic Spectra of the Sun

    Scientists have measured both the blackbody spectrum and also the atomic spectra of various elements in the Sun. How do they distinguish between the two and filter out the light from either one?
  39. T

    I Different Blackbody Intensity Peaks, how do we measure it?

    For the Blackbody Spectrum, there are two versions of the formula, one for wavelength and the other one for frequency: The peak intensities for both occur at different wavelengths (or frequencies).How do scientists measure the spectral radiance of blackbodies? Are there TWO types of...
  40. T

    Converting between λ and ν for Blackbody Radiation?

    Forgive me for this stupid question, but how do I convert between and I tried c = νλ but that doesn't work. This is the Rayleigh Jeans Law by the way.
  41. S

    Fit blackbody spectrum to data in python

    Hi! I have to fit a blackbody spectrum to some data points. The y-axis is in mJy and the x-axis is in log_10(freq). My code looks like this: from __future__ import division import matplotlib.pyplot as plt import numpy as np from scipy.optimize import curve_fit h = 6.63*10**(-34) c =...
  42. Ryaners

    (Statistics) Blackbody spectrum in terms of wavelength?

    This is a question about transforming a probability distribution, using the blackbody spectrum as an example. Homework Statement An opaque, non-reflective body in thermal equilibrium emits blackbody radiation. The spectrum of this radiation is governed by B(f) = af3 / (ebf−1) , where a and b...
  43. S

    Jansky for blackbody radiation

    Hi! I hope this is the right place for this question. I have a plot of mJy vs frequency and I want to fit a blackbody plot to it. I just don't know how to convert the y-axis correctly to use the formula for the fit. Any help is appreciated.
  44. mukul

    Black Body Radiation and ambient temperature

    Homework Statement In a dark room with ambient temperature T0, a black body is kept at a temperature T. Keeping the temperature of the black body constant (at T), sunrays are allowed to fall on the black body through a hole in the roof of the dark room. Assuming that there is no change in the...
  45. G

    Blackbody Radiation: Ideal Emitters at Room Temperature

    Hi, Very basic question. Blackbodies are ideal emitters: at every frequency, they emit an amount of energy equal to or greater than any other object at the same temperature. Furthermore, they were named blackbodies since, AT ROOM TEMPERATURE, i.e. 300K, whatever radiation they re-emit (after...
  46. Hyo X

    KT (energy) vs Blackbody Radiation for molecule in a box

    I'm trying to understand the meaning of kT (energy) in molecular systems, how to define the temperature of an individual molecule, and how a molecule receives thermal energy or dissipates thermal energy. Here is my 'gendanken': One molecule is floating in a box, in vacuum. The temperature of...
  47. A

    Confusion With Blackbody Radiation

    A blackbody is a theoretical object that perfectly absorbs all the light that falls on it. From what I understand this is an ideal situation and does not actually exist in reality. Certain objects are close to being a blackbody but they do not absorb 100% of the light that hits it (i.e. some...
  48. Raman Choudhary

    Understanding Blackbody Radiation in Quantum Mechanics

    I'm trying to start understanding quantum mechanics, and the first thing I've come across that needs to be understood are black bodies. But I've hit a roadblock at the very first paragraphs. :( According toWikipedia: A black body (also, blackbody) is an idealized physical body that absorbs all...
  49. C

    Wavelength/Frequency inside a cavity

    http://www.cengage.com/resource_uploads/static_resources/0534493394/4891/Ch03-WebAppendix01.pdf I've been doing some reading and I came across a mathematical step that is confusing me. Could someone explain to me how they get from equation 3.45 to equation 3.46 in the link that I have posted...
  50. 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.