What is Blackbody radiation: Definition and 113 Discussions
Black-body radiation is the thermal electromagnetic radiation within or surrounding a body in thermodynamic equilibrium with its environment, emitted by a black body (an idealized opaque, non-reflective body). It has a specific spectrum of wavelengths, inversely related to intensity that depend only on the body's temperature, which is assumed for the sake of calculations and theory to be uniform and constant.The thermal radiation spontaneously emitted by many ordinary objects can be approximated as black-body radiation. A perfectly insulated enclosure that is in thermal equilibrium internally contains black-body radiation and will emit it through a hole made in its wall, provided the hole is small enough to have a negligible effect upon the equilibrium.
In a dark room, a black body at room temperature appears black because most of the energy it radiates is in the infrared spectrum and cannot be perceived by the human eye. Since the human eye cannot perceive light waves below the visible frequency, a black body at the lowest just faintly visible temperature subjectively appears grey, even though its objective physical spectrum peak is in the infrared range. The human eye essentially does not perceive color at low light levels. When the object becomes a little hotter, it appears dull red. As its temperature increases further it becomes bright red, orange, yellow, white, and ultimately blue-white.
Although planets and stars are neither in thermal equilibrium with their surroundings nor perfect black bodies, black-body radiation is used as a first approximation for the energy they emit. Black holes are near-perfect black bodies, in the sense that they absorb all the radiation that falls on them. It has been proposed that they emit black-body radiation (called Hawking radiation), with a temperature that depends on the mass of the black hole.The term black body was introduced by Gustav Kirchhoff in 1860. Black-body radiation is also called thermal radiation, cavity radiation, complete radiation or temperature radiation.
I can't seem to find a definitive answer to my questions surrounding this topic, every textbook I read is rather vague. When Max Planck scratched the surface of quantum mechanics he introduced the idea of quantisation of energy in blackbodies. Could someone please explain how exactly a blackbody...
Homework Statement
Expression for the entropy and internal energy of black body radiation.
Using the below relations:
Homework Equations
Total free energy for black body:
$$ F = (k_b TV/\pi^2) \int k^2 ln[1-exp(-\hbar ck/k_b T)]dk $$
Relationship between partition function and internal...
Homework Statement
Show that the ratio of the blackbody fluxes from a star at two different frequencies (i.e., a color) is measured, then, in principle, the surface temperature of the star can be derived, even if the star's solid angle on the sky is unknown (e.g., if it is too distant to be...
The Stefan–Boltzmann law :
J = kT^4 where k is Stefan–Boltzmann constant.
but for non black body ( or grey body)
the equ becomes
J = εkT^4
where 0<ε<1
can anyone explain y a grey body emit less radiation when compare to the black body at same T? It can found the ans in wiki or the...
Homework Statement
A rectangular metal plate measures 0.20 m long and 0.3 m wide. The plate is heated to a temperature of 1,433 K by passing a current through it. Assuming that it behaves like a blackbody, how much power does the plate radiate under these conditions?
Homework Equations...
Homework Statement
The blackbody radiation equations of state are E/V=aT^(4) and P=(1/3)(E/V). From the fundamental state derive a quantity involving only P and V that remains constant when this radiation undergoes a reversible adiabatic transformation.
Homework Equations
Maybe...
I have been reading a lot of stuff on blackbody radiation and the ultraviolet catastrophe.
Here is what I have so far.
The ultraviolet catastrophe arises from the classical electrodynamics predicting an infinite amount of energy from a blackbody having any temperature.
As far as I have...
Homework Statement
The problem (18.1a) can be found here:
http://www.suagm.edu/umet/paginas/dbacelo/chem464/scan-probcap18-levine-pag1.pdf .
For reference, the equation (referred to as 18.2 in the problem statement) for the frequency distribution of blackbody radiation is give as...
This might be more of a mathematical question than a physical one. But I am taking a Quantum Mechanics course and the book starts out by introducing the equation for the energy density of radiation from a black body. They then integrate this expression over infinity to find the total energy...
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...
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...
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...
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...
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...
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...
Homework Statement
Raising the temperature of 1cm^{3} of water (1 gram, water has a heat cap. of 4.2 J/g*K) using energy from a cavity filled with black body radiation. The water is to change from 299K to 300K. The radiation is initially at 450K.
If the cavity has a volume of 0.1 m^{3} how long...
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...
"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)...
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...
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...
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...
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...
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...
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?
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...
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 =...
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
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...
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?
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...
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...
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...
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...
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...
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) =...
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...
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...
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...
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...
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...
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...
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}...
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}...
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...
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...
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...
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...
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...
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?