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Questions tagged [thermal-radiation]

The temperature-dependent emission of electromagnetic waves. Combine this tag with [tag:thermodynamics] for a macroscopic view or [tag:quantum-mechanics] for a microscopic explanation.

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47 views

Lasers to direct waste heat?

I've been thinking about ways to cloak ships in space, since they all are warmer than their background they can be seen in the IR spectrum (given a large enough telescope). So I was thinking, could ...
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Is Sun brighter than what we actually see?

I learned from that plasma can reflect radiations of frequency less than that of its own oscillations. If so, considering the plasma in Sun's atmosphere, it should also reflect solar radiations. That ...
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Heating due to sun's radiation [duplicate]

It is commonly said that heat received from sun is due to the infrared part of solar radiation. But from Wikipedia i found that most of the energy is received from visible light. Then why do we say ...
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Light Intensity vs Power for a Small Incandescent Lamp

Today I did a quick experiment where I measured the light intensity of a small incandescent bulb for a range of voltages (0 to 31V). I also measured the current. I expected to see the light ...
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Laser Detection Paint

Does there exist a paint which if a pulsed laser of ~1500-1600nm was fired at it it would emit visible light or IR Radation (3-5 micron)? I have seen up-converting inks and paints but I can't find ...
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1answer
71 views

How much did the universe expand since recombination time? [closed]

Assuming adiabatic expansion, and a temperature at recombination time of $2700^{\circ} C$, how much has the universe expanded if the temperature today is $2.7K$? My idea is to use $\frac{dQ}{dt}=A\...
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Emissivity not accounted for in Equilibrium Temperatures of Planets

I'm trying to understand how to calculate the Equilibrium Temperatures of Planets. This is the beginning of my working for it... However, whenever I look at external sources, (like this Princeton ...
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What is Kirchoff's thermal radiation law? [closed]

Is Kirchhoff's law true even if body is not thermal equilibrium with surrounding. And according to Kirchhoff's law absorptivity is equal to the emissivity of body.
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Violation of Stefan's law when shining a light on a black body

Suppose there is a black body in a dark room and the room temperature is constant. Now a ray a light is shone upon it. Since a black body absorbs all radiation that falls upon it, it must absorb more ...
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interference of blackbody radiation with medium

We know that radiation of blackbody is unpolarized. And also its phase are random. So I thought that electromagnetic waves from blackbody wouldn't be interfered. But what if there's a medium? For ...
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Do photons exist at all possible wavelengths? [duplicate]

My question refers to Photon flux spectrum diagrams. The diagram shows the number of photons at different wavelengths. My question is whether the graph is granular or continuous. Do photons exist at ...
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Why is the Sun approximated as a black body at ~ 5800 K?

Apparently spectral solar radiation is approximated by a black body at 5800 K. The spectral black body distribution (Planck distribution) is shown below (from Incropera, Fundamentals of Heat and Mass ...
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How come we can treat space as a blackbody?

I thought that the condition to be a black body was that an object had to absorb all incident radiation without reflecting any. My understanding is, electrons in atoms absorb the incoming radiation. ...
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How to add protective enclosure around exposed thermal camera

First off, let me state that I'm not entirely sure this is the right forum for this question. This was the only section I found multiple questions pertaining to thermal imaging, however, so I thought ...
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Thermal radiations

everything having temperature above 0 k emits radiation. But those radiations depend upon temperature of the body and not on nature of the body. Why thermal radiations are not material dependent?
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How much conduction, radiation and convection influence in a thermos flask?

It has been thoroughly repeated that thermos flasks prevent heat loss by minimizing heat conduction, radiation and convection from the bottle to its ambient. This has been answered in this SE. However,...
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How many degrees Celsius/Fahrenheit can be reached in 1 square meter area using magnifying glasses?

The magnifying glasses are each 10 meters in diameter, their collected light is concentrated via mirrors into a 1 square meter spot. Is it possible to get a temperature higher than 2000 degrees ...
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Heat transfer on Mars

SCENARIO: I have been tasked with finding the heat transfer across a composite wall on Mars. On the left of the wall there is air maintained at Earth atmospheric conditions and room temperature (101 ...
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Black hole absorbing photons?

If a black hole has a radius that is not that much smaller than the wavelength of light emitted by the sun, and is at the same temperature, shouldn't it be able to absorb photons as well as emit them? ...
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1answer
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How many $\text{watt}$ are required to obtain $500°$$\text{C}$? [closed]

I am using two $1000$ $\text{watt}$ heating elements in a cylindrical chamber of volume $7500$ $\text{cm}^3$ with a limited opening at two sides. The cylinder has a radius of $6$ $\text{inch}$. How ...
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Is thermal coupling to a too cold thermal background impossible?

In (Krauss & Starkman 1999) the authors critique (Dyson 1979), and in one section argues that in the very far future cooling will become impossible because thermal contact with the cosmic ...
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1answer
147 views

Emissivity and absorptivity relation question

This question is related to another discussion here about emissivity of bodies. What interested me is to verify what extend the wavelength between emissivity and absorption is conserved at thermal ...
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What is the reason for the emission of electromagnetic waves from the sun? [duplicate]

What is the reason for the emission of electromagnetic waves from the sun and why curve its in about the visible light is maximum?
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How much change in Earth's orbital distance to change average temperature by 1°C?

How much closer/farther would the Earth need to be to/from the Sun to effect a $1 \sideset{^{\circ}}{}{\mathrm{C}}$ increase/decrease in average temperature?
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What actually causes blackbody radiation? [duplicate]

I've spent a fair bit of time reading up on answers detailing exactly how blackbody radiation is created but I have seen two different explanations for how it is created. The Kinetic Energy (KE) of ...
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Why do we take the derivative of the number of modes with respect to frequency (Rayleigh-Jeans)

Source: http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/rayj.html We arrive at this result: $$N(\nu)=\frac{4}{3}\frac{\pi L^3\nu^3}{c^3}.$$ So now we have a function representing the number of ...
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Questions on derivation of Rayleigh-Jeans equation

So it starts by assuming a cube cavity with length $L$ that acts approximately as a black body. [note: the cube length is $a$ but I'm using $L$ instead] Standing waves are formed inside the cavity, ...
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Why is methane considered an important greenhouse gas?

Methane’s absorption bands are centered at 3.2 and 7.2 microns -- far off the peak of the Planck spectrum for a 290 K blackbody near 16 microns. Moreover, its absorption bands overlap with the water ...
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Which emits more radiation - a warming blackbody or a cooling blackbody?

My intuition says a cooling blackbody would emit more radiation as any body that emits radiation is itself losing that energy, causing its internal temperature to decrease.
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Factor of 4 discrepancy between integral of Planck's law vs Stefan-Boltzmann law

Consider an absolutely black sphere with unit surface area and temperature $T$. Planck's law states for the power emitted per unit area of the emitter per unit solid angle of emission per unit ...
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Infrared pass material thermal camera

I have a situation in which I need to use a thermal camera in a rain enviroment, the camera is mounted on a gymbal and can rotate 360 degrees, so I tought it may be an option to build a dome in order ...
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2answers
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Black body problem

What will happen if a black body at room temperature is dropped into a furnace?? What will it look initially and how in the steady state.? I guess initially it should be black as it is absorbing a ...
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1answer
45 views

If the Sun had a larger surface temperature how would that affect its appearance to us in the sky?

I thought about this when I came across wiens displacement law which says the higher the temperature, the lower the peak wavelength. If the sun was a lot hotter, and its peak wavelength wasn't in the ...
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1answer
215 views

Is the quantum harmonic oscillator energy $E = n\hbar\omega$ or $E = (n + 1/2)\hbar\omega$? (Feynman Lectures)

Please, read the whole question. I've discussed a few contradictions and so far have not found an explanation for them. I was reading The Feynman Lectures on Physics (vol. 1), the part where he talks ...
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2answers
58 views

Why does a dense, hot, opaque body produce blackbody radiation?

I read from a textbook that a dense, hot and opaque body produces black body radiation, for example the heated filament of an incandescent light bulb. Why is it so? The explanation given is if a ...
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1answer
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Given the radiation emited from a black body in a range of wavelength, how to find the temperature of such body?

For a black body that emits a radiance of $5\times 10^5 Wm^{-2}$ between the wavelengths $\lambda_1=520nm$ and $\lambda_2=550nm$, find the temperature of the given body. I think we want to work with ...
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1answer
158 views

How to develop a model for cooling rate of steel tube in air?

I am trying to develop an equation to determine the cooling rate of a steel tube in air. I'm using Fourier's Law, Stefan-Boltzman Law, Newton's Law as well as the specific heat capacity equation. The ...
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1answer
221 views

Derivation of entropy for ideal vs. photon gas

Here's the standard way of deriving the entropy of the ideal gas (see e.g. here): $$ dQ=dU+PdV=C_VdT+\dfrac{NkT}{V}dV $$ $$ dS = \dfrac{dQ}{T} $$ Integration of the latter gives the correct result. ...
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Unit of Spectral Emissive Power

I understand Spectral Emissive Power as the total amount of energy carried by photons having the same wavelength (energy), and it has the unit of (W/m2.um). I could not grasp the physical ...
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1answer
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optimal spacing for stove pipe heat shield

Note: Question is DIY'ish, but analysis seems pretty theoretical, if less so than typical for this site (I'm new to it). Should I move to "engineering" site ? The flue for my wood stove transitions ...
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1answer
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How/Why does heat radiation work, atomically?

In this question the relationship between characteristic spectral lines of elements and the apparently continuous emission of blackbody radiation was examined. It was suggested in the answer that ...
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2answers
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How does the heat capacity of an object come into play in thermal radiation?

So say there's a cube in space acting as a blackbody. Each side is 2 metres. Initial cube temperature is 400 Kelvin. Mass is 15 kg. Say the heat capacity is 500 J/kgK. How would that affect thermal ...
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Treating stars and planets as a blackbody determine the temperature of a planet that has EM radiation incident upon it from a star

Consider the following question: Power emitted by the star via the Stefan Boltzmann law is as follows: $$4\pi\sigma T_s^4 r_s^2 $$ However the power will drop over distance and so we can say that ...
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Why should blackbody radiation emit all the absorbed energy?

So it is said that a perfect blackbody absorbs all the energy and then emits all of that in the form of blackbody radition. Why should it emit all the absorbed energy? Why can it not emit some and ...
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How do I get energy distribution of radiation (Planck's Law) in terms of frequency?

I have $$u(\lambda) d\lambda = \frac{8\pi hc\lambda^{-5}}{e^{hc/\lambda kT} - 1}d\lambda$$ I want this in terms of frequency. I can start with $\lambda =c/f$ but I will end up with $u(c/f)$ rather ...
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Planck equation in terms of frequency [duplicate]

Given $$u (\lambda) d\lambda = \frac{8 \pi hc \lambda ^{-5}}{e^{\frac {hc}{\lambda k t}}-1}d\lambda,$$ I want to convert this into frequency form $u(\nu)d \nu$. Basically, I want to covert the amount ...
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Thermal equilibrium of objects in deep space

I've read somewhere (not a reliable source though) that an object in space and far enough away from any heat source other than the CMB would eventually cool to 3K, the CMB's temperature, but no colder....
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Could food be used as heating element?

Could we heat food by flowing electricity through it as a heating element?
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Is the *measured* temperature inside a hollow black body the same if we place a thermometer inside it or let it make contact with the body itself?

Imagine the inside of a black body shell with non-zero thickness. Inside the shell, there is photon gas produced by the shell [at a temperature much higher than zero, say $1000(K)$]. Is there is a ...
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1answer
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Will a black body placed somewhere around the Sun obtain (eventually) the same temperature as the Sun?

Suppose we look (above the Earth's atmosphere) at the wavelength ($=\frac c f)$ spectrum emitted by the Sun: This shows that the Sun is approximately a black body with a temperature of about $5525(K)....