I have read in thermodynamics that a body with high temperature radiates electromagnetic radiation. Does the frequency of waves increases with temperature? What is the mechanism? How does temperature does that?
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By conservation of energy, the solid is left in a lower energy state following emission of a photon.  Clearly absorption and emission balance at thermal equilibrium, however, thermodynamic equilibrium is a statement of the mean behaviour of the system, not a statement that the internal energy is constant on arbitrarily short timescales.  The energy has to come from somewhere during emission, and go somewhere during absorption.  Energy in a solid can be stored as kinetic and potential energy of electrons and nuclei, either individually or in collective modes such as phonons and plasmons.  In thermal equilibrium energy will be stored more or less in various forms depending on the temperature and material.  However, even if most of the thermal energy in a particular solid at temperature $T$ is stored in the form of phonons, it could be that phonons primarily interact with light indirectly via electrons, e.g. a phonon excites an electron in a phonon-electron interaction, which can interact with light via the EM field.  Given that light is an EM field, it makes sense to me that it is emitted and absorbed by charged particles.  The electron-photon interaction is probably dominant for visible and ultraviolet light, given that metals are opaque, while semiconductors and insulators are transparent to (visible and UV) light with energy lower than their bandgap. However, once you get into energies in the IR and below, or X-rays and above, other mechanisms apparently take over.  For example, on the high-energy end of the spectrum I've heard that gamma rays can interact directly with nuclear degrees of freedom, which is reasonable considering that gamma rays are emitted during a lot of nuclear reactions.  A review of  might give clues to important light-matter interactions over a broad range of wavelengths.  Whether all of the these processes are involved in blackbody emission is a somewhat different question.  What physical processes mediate energy transfer during blackbody emission, and in which energy ranges are the various processes dominant?
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My  wasn't specific enough. I'll try to be more specific.  Let's imagine we have a hot body let's say 6000K hot that emits lots of thermal radiation. Let's assume 1kW of radiative power falls on on a m². Now if we use a lens with 1m² area, we can focus this 1kW into a smaller area which heats up until it radiates back the incoming power.   Now by using a lens with a smaller focal length, the image of the hot body can be smaller. So we can concentrate the radiation into a smaller area. The smaller the area you concentrate the power, the hotter that area becomes isn't it?   But I read in lot of places that this hot spot cannot be hotter than the body that that provides the energy, so in our example it cannot be hotter than 6000K, because it would violate the laws of thermodynamics. So what happens if you keep decreasing the focal length and size of the image? Maybe you cannot shrink the hot spot smaller than some size due to the wave interference and diffraction, thus ensuring this constraint?
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We know how to define the , as answered in this question. After watching the Sixty Symbols video about , where the temperature and resulting black body radiation is a crucial element, I wondered how objects made (almost) entirely of neutral particles can radiate electromagnetic waves, since radiation is caused by acceleration of charged particles (at least in classical physics). I can imagine a few possible answers, but I would like the expert's statement:  Neutrons can be excited and radiate when relaxing Neutron stars aren't entirely neutral. The tiny fraction of protons and electrons radiate Neutrons are made up of charged quarks, and the quarks radiate  I believe this question is different from &quot;Are neutron stars transparent&quot; because I'm asking about the generation of black body radiation, not the lack of absorption of any radiation.
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This asked:   And also this:     The second question is contradictory; maximum frequency -> minimum wavelength.   I am asking the very opposite;    What is the minimum frequency and maximum wavelenght of electromagnetic radiation?   The lowest measured/defined seems to be 3 Hz;  Which means a wavelenght 1/3 of the speed of light; ~100 000 000 m.   But this can't be the physical limit for the wavelenght. Does such a physical limit for the wavelength exist? (Similar limit like the speed of light is for velocity).
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I know that electromagnetic waves induce electric currents in conductors and that's the basis for radio, wi-fi etc.  I also know that light is also an electromagnetic wave. So, can light induce a current in a conductor (like a metal wire? or a coil?). And, if the answer is yes, is the same visible for other high-frequency waves (X-rays, gamma)?  I heard about the photoelectric effect, but it seems related to the particle theory of light (photons transferring their energy to electrons).  So, do high-frequency electromagnetic waves generate electric currents? Is it possible to measure them? Is the skin effect relevant here?
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How does the color of a car influence its inner temperature change over time when parked outside in windless, hot and sunny regions?  I know what's the common idea about that: black cars are supposed to build up high temperatures faster because black objects absorb radiations instead of reflecting them off. Is it true? And then, what happens during the steady state? Some claim the black car will stay hotter, some others claim it will be the same whatever the color. Do all cars have the same temperature inside in the long run, or are some of them hotter?  I've Googled it but the best I could find was . It claims to be based upon ~20 articles. But there's not even a mention of absorption or emission spectrums. A priori, a black car could absorb all visible radiations and emit it as infrared while reflecting all of the Sun's infrared. And it's still only qualitative. I've found nothing on Physics stackexchange, the closest being .
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I saw  Nature article today, which cites e.g. . And it makes no sense to me. The temperature of a collection of particles is the average kinetic energy of those particles.  Kinetic energy cannot be less than zero (as far as I'm aware), so I don't understand what this article is trying to say, unless they're playing around with the conventional definition of &quot;temperature&quot;. The only thing I can think of is if you have something like this: $$\frac{1}{kT} ~=~ \left(\frac{\partial S}{\partial E}\right)_{N,V}$$ And they've created a situation where entropy decreases with increasing energy.
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I am near the end of the book  by Steven Weinberg. I am reading it in order to get a better picture of the early universe in the Big Bang model. But one thing I am having trouble with and wanted to get some clarity. Namely: Question What is meant by &quot;temperature&quot; if the  in the Big Bang model was mostly radiation, or pre-inflation, something more basic than radiation? Explanation Below I will try to explain why I am getting confused. Please let me know if it needs clarification (but that's also part of the question, I haven't been able to clearly understand temperature in this context yet). Since the  is about the movement of massive particles: $$T   =   {m\overline{v^2}\over 3 k_B}$$ How do you think of temperature when there is no mass, or when there is mostly radiation in the very early universe? Here is where I get confused.. Radiation is moving at the speed of light $c$, and since the speed of light is constant, and since radiation doesn't have mass, it seems like the universe would be &quot;maximally&quot; hot (particles moving at the speed of light). But intuitively that's not true because the temperature formula requires mass. But then if the early universe was mostly radiation, how does it have a temperature? If mass slowly increased to be the majority ~400,000 years after the Big Bang, it seems the temperature would increase rather than decrease. But then taking into account the expansion, it makes sense that even if more matter was forming, the temperature would decrease. But how could it decrease if there was less matter before? To summarize, in trying to pinpoint temperature, I seem to be stuck in cyclic reasoning and am looking for a better way of understanding it. I am working toward learning the purely mathematical form of these theories, but have a ways to go. For now, looking for a description to broadly make sense of &quot;temperature&quot; during the evolution of the few moments after the Big Bang. Examples of how &quot;temperature&quot; is used In the book above, Steven Weinberg talks a lot about the temperature and radiation of the early universe, and how the early universe was dominated by photons/radiation, and ~400,000 years later became dominated by mass-particles when the temperature cooled. Some other quotes from the web include:  In the first moments after the big bang, the universe was extremely hot and dense. As the universe cooled, conditions became just right to give rise to the building blocks of matter – the quarks and electrons of which we are all made. () ...the temperature was so high that the four fundamental forces ... were one fundamental force. () ...the temperature of the universe was still too high to allow quarks to bind together to form hadrons. ()
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My understanding was that all objects emit light of continuous spectrum when hot ( black body radiation) but then you see discreet wavelengths in spectra lines and I am confused. I know I am making a mistake somewhere but not able to point it
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I spent some time studying about temperatures and color of objects. It turns out that as we heat something it turns to red, then yellowish white and if we heat it more it turns to bluish-white.  Like we can say a blue star is hotter than a red star. But why isn't it the same with flames?  Blue flame isn't always hotter than a red flame. It's just the chemistry of it all.   I mean I know the electrons jump from one orbit to another and gets into excited state when heated.  But the questions in my mind are:   If one object appears blue and another red, that doesn't always mean that the blue object is hotter than red. Is that correct? If yes then how is it so? I am sorry but I'm bit confused over it. With the "Color Temperature" concept on Wikipedia. They say 1,700 K to match a flame. But 15,000–27,000 K for a clear blue poleward sky.   This confuses me. The sky appears blue, does that mean its hot? But it isn't right? It's colder. I am not sure if I'm able to frame it properly. It's a bit confusing to me.  Is this something like a glowing object vs reflecting object? The sun is white but Earth is blue. But sun is hotter than the earth. It's the sun radiating light but earth is just scattering it.  But the surface temperature of sun is 5778K while the Wikipedia states our sky is about 15000K-27000... I know there's something I'm missing here. I'm hoping someone can tell me what is it.
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As I understand it, Stars emit visible light, OBAFGKMRNS, in the range of $10^3 - 10^4 K$. Yet materials such as steel emit similar frequencies at much lower temps; red is around 800K. Why the difference? I thought black body radiation applies to all materials and environments. I am an interested amateur.
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My understanding is that a black hole radiates ~like an ideal black body, and that both photons and massive particles are emitted by Hawking radiation. So for a low temperature black hole, photons are emitted according to Planck's law, but the peak of this spectrum shifts to higher frequencies as temperature increases.  For sufficiently large temperature, does it follow that massive particles of equivalent energy are also emitted, without preference to other particle properties? If not, I'd be interested to know what physics are involved in determining the form in which the black hole's energy is radiated away.
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I am confused on why would the propagation speed of any EM waves at ANY frequency is constant in the free space (vacuum) but they seem to disperse in any other materials as the propagation speed of EM waves in other materials depends on the frequency? Can someone give me a good proof of this phenomena with appropriate formulas?
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Okay, so everywhere I've read, I hear the main difference is the requirement of a medium. But for example, if you take the case of heat 'radiating' from  a red-hot iron, isn't that actually  and not ? I mean, isn't the temperature difference between the surrounding air and the iron causing the air to gain heat?
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During emission spectrum $$\Delta E=h\nu,$$ where $\nu$ is the frequency. All books write that it is the frequency of photon, but photon is a particle and not a wave.  More than that what this frequency actually is? Is it the frequency at which energy packets are released?  This wave and particle nature is causing conflict!  What is the meaning of frequency of photon? I mean it is a particle and not a wave and frequency is a physical quantity associated with a wave.
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A  is a device that concentrates the sun's light on a small point to heat it up to high temperature. One can imagine that in the limit of being completely surrounded by mirrors, your entire $4\pi$ solid angle will look like the surface of the sun, at about 6000K. The target will then heat up to 6000K and start to radiate as a blackbody, reaching thermal equilibrium with the sun.   The question is: is there any way to surpass this temperature, perhaps by filtering the light to make it look like a BB spectrum at higher temp, then concentrating it back on the target?
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Because an object's temperature is inversely proportional to the wavelength of blackbody radiation which it emits, physicists have theorized the existence of  at around $1.4×10^{32}$ K.   Does this imply that temperature must be discrete? Meaning, the temperature could never be $1.0×10^{32}$ K, because that would imply a wavelength which is a non-integral multiple of the Planck length.  If it does imply this, does that mean that a system, if approaching the Planck temperature, and given more energy, would suddenly and radically change states from $0.7×10^{32}$ K to $1.4×10^{32}$ K? (perhaps much like a )  Pardon me if this question is bizarre, as I am not a physicist by profession (except maybe of the armchair variety).