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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Photon pumping in Laser

Let's consider a ring laser where the laser must pass through the gain material before it is sent toward a partially reflective surface $\ R=1-T $. The other mirrors are perfect reflectors with $\ R_1=...
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How can a solid be thermally transparent?

Thermal cameras designed to image wavelengths between 7-14µm (which is the peak for blackbody radiation around "room" temperatures) use lenses made of germanium, zinc sulfide, zinc selenide, or ...
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Is there a difference in the infrared absorption spectrum of a greenhouse gas when pure and when mixed with non-greenhouse gases?

According to the standard IPCC greenhouse climate change hypothesis a doubling of the preindustrial CO2 concentration of 285 ppm in the atmosphere - the current value is 405 ppm - would lead to an ...
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Why isn't cosmic microwave background radiation on Earth drowned out by thermal noise

The Penzias/Wilson experiment, which detected the CMB, doesn't sound feasible given thermal noise. When Penzias and Wilson reduced their data they found a low, steady, mysterious noise that ...
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Calculating new temperature of an object when air temperature changes

I'm trying to calculate the new temperature of an object when the air temperature around it changes, given a period of time. Basically I get periodic readings from an air temperature sensor in a ...
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1answer
269 views

What does a hot, optically thin gas *look* like?

In another question I tried to answer what a sample of the Sun's photosphere or core would look like, if it could be brought into the lab. Here is a broader question - if I have a small inert ...
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What is the radiation field of a black body with temperature $T$ translating at speed $v$?

Suppose I have a spherical black body at temperature $T$ with radius $R$. When stationary in a vacuum at zero Kelvin, it produces an incoherent radiation field according to Planck's law $$ B_\nu = \...
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Rigorous treatment of blackbody radiation

I cannot understand the logic behind treating each normal mode of an electromagnetic radiation as a harmonic oscillator in the treatment of blackbody radiation. What do I need to know to treat it ...
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80 views

Reaching equilibrium in a blackbody and light-matter interaction

Suppose we have a metallic cavity maintained at a fixed temperature. Suppose we start with any distribution of radiation that is not in equilibrium with the container. Gradually, when the equilibrium ...
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306 views

Chemical Potential and Blackbody Radiation

What is the physical significance of the fact that the chemical potential of the photons in a box of volume V is zero ?
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black body simulation

black body radiation is typically understood from Planck's argument of light resonance in a box, from which the density of states is computed. Now, suppose I want to simulate a black body ...
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745 views

Does coffee with cream cool slower than black coffee?

At Modernist Cuisine, Nathan Myhrvold states that "Coffee with cream cools about 20% slower than black coffee". He then gives three reasons, two of them related to radiation and the third one to the ...
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A problem concerning the change of temperature and spectrum of a filament

The spectrum of a filament has been given before, the left one having the lowest temperature, the middle with a medium temperature and the right one with the highest. My question is this: Why does the ...
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Can Earth's mass increase due to sunlight? With how much?

Asking this just out of curiosity: I am no physics buff but considering the particle nature of light, each photon has energy (however small it may be). Given this, can the earth's mass increase given ...
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thermal equilibrium in deep space

Imagine an object- say, a sooty asteroid- in deep space, far enough from any star to receive only negligible radiation from it. We make it sooty so as to make it a good absorber of radiant energy and ...
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Emissions from a Blackbody and the UV Catastrophe

Recently I found myself becoming confused over a topic that I thought I had previously understood. In a theoretical blackbody which reaches thermal equilibrium the energy absorbed is equal to the ...
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68 views

Why does the emissitivity of metals increase (a lot) with temperature?

If we take a piece of metal (could be silver, nickel, platinum, copper, etc.) at room temperature, its emissivity is relatively low compared to that of a black body: about 0.05 vs 1.00. However if ...
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Does same material with different colour will ended up in same temperature if both of them receive same thermal radiation?

"Every good heat radiation absorbent is a good emitter." Suppose I have two identical material with different colour, placed in different room, receiving same and only thermal radiation. My ...
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Shortest path to understanding black body radiation

I'm a mathematician studying quantum mechanics. I noticed I lack some statistical physics prerequisites. However, because I am in a rush, I'd like to learn just as much of it as I need to understand ...
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What is the physical limit of thermal insulation?

If we consider a container (at very low) temperature $T_0$ surrounded by some passive structure, in turn surrounded by an environment at temperature $T_1>T_0$, there will be an inward heat flow ...
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What portion of the total power emitted by the Sun comes from photons?

We usually talk about luminosity, which is the total power from emitted photons. The other sources are neutrinos and the solar wind (which includes protons and other particles). I assume that most ...
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Does the black-body radiation formula (Stefan-Boltzmann) change if the photon turns out to be massive?

The black-body radiation formula, the Stefan–Boltzmann law, is derived treating photons as massless. Questions: What would the Stefan–Boltzmann law look like if derived assuming that photons have ...
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Why gravitational waves are not part of thermal phenomena?

Electromagnetic waves are part of thermal phenomena in the form of thermal radiations. But why gravitational waves do not show up as a thermal phenomenon, for example, why gravitational waves do not (...
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1answer
231 views

Wien on Temperature and Entropy

As far as I understand, Wien defines entropy, which he uses in his derivation, as $$ S = \text{v} \int\limits_0^{\infty} \varphi(\rho, \nu) d \nu, $$ where $\text{v}$ is the volume occupied by ...
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427 views

The derivation of the Planck distribution

I am trying to understand the Planck distribution and black body radiation. In the Wikipedia derivation of the Planck distribution, the photons confined within a cubic box, are emitting from and ...
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57 views

How does a body heat up?

Suppose a block of metal B is in a vacuum chamber at room temp (27°C) and we irradiate it at 77*C Can anyone explain what happens step by step in B, and how actually its temperaure rises? DO the ...
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What is the relation between KE of a nucleus and thermal radiation?

In a monoatomic gas the KE of an atom is 3/2 kT, so at room temp (300K) its speed is roughly 15 Km/s, is that right? Can you tell how to figure out the parameters of a nucleus in a lattice https://...
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Inverse pair production with hohlaraum as photon target; is this particular experiment going to be carried out?

The IFLScience article Scientists Work Out How To Make Matter From Light describes photon-photon collisions producing pairs of particles, and the Nature Photonics Letter A photon–photon collider in a ...
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297 views

Planck law derivation, number of modes, average energy

I am just starting learning quantum physics, and I am starting from old quantum theories and from black body radiation, but I am getting in trouble to understand Planck law derivation. I am too ...
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Planck's theory question

Could you please tell me why Planck's theory ceases to be valid when the sizes of the bodies and/or their separation distances are comparable to, or smaller than, the wavelength. My professor told me ...
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130 views

Maximum temperature possible on earth

What is maximum temperature that can we have on earth on a single day? Lets say an air mass is static over an area and there is no way for air mass to move, sun warming it up would increase ...
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1answer
414 views

How can I apply Planck's law to calculate the photon energy density of a warm room?

I am trying to calculate the rate at which photons of any particular frequency will pass through a volume in a room illuminated by black body radiation only. I've found a couple of starting points ...
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81 views

Car Heating due to the sun: comparison of different contributions

Let's imagine that we need to choose a car in order to minimize the Temperature that the inner part of the car will reach after some hours under the sun. We know that two factors (if we suppose that ...
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In theory, Is it possible irradiate a blob of matter -for instance- with microwaves in order to that matter emit more energy than has received?

Suppose that you want coerce a block of matter to lose his energy. I mean thermal energy. This should be done by irradiation of more energy. The final thermal energy should be lower that the original ...
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Radiative cooling time for a black body

Okay, so this is confusing me a bit. How can I calculate the time it takes to cool a perfect black body from an initial temperature $T$ down to equilibrium temperature (say, 3 K for space)? I know ...
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356 views

What happens when a lower energy photon strikes a higher energy atom?

I understand that when a higher energy photon hits an atom it could elevate an electron and add energy to the atom, but what happens if a lower energy photon strikes a higher energy atom? If it doesn'...
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Which would keep you cooler? A black canopy tent or a white canopy tent?

I was outside recently at a function and they had two colours of canopy tent set up: black ones and white ones. I was wondering which one would be cooler to sit under. The white one would reflect the ...
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How quickly does water lose heat by thermal radiation?

Let's say I hold in space room-temperature water, shadowed so no radiation is emitted onto it and surrounded by vacuum (let's assume it won't boil even with the low pressure). I assume the surface ...
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Drag force on mirror immersed in blackbody radiation

In Einstein's "On The Present Status of the Radiation Problem" he mentions a thought experiment where A mirror of mass $M$ and area $A$ that is perfectly reflective in the range of frequencies $\nu$ ...
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Difference between spectral radiance and specific intensity for blackbodies

The spectral radiance of a blackbody is given by Planck's law $$B_{\nu}(\nu, T) = \frac{2 h\nu^{3}}{c^{2}}\frac{1}{e^{h\nu/k_{B}T}-1}.$$ It has units W$\cdot$sr$^{-1}$$\cdot$m$^{-2}$$\cdot$Hz$^{-1}$. ...
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Does the Planck length imply that the smallest different between any two wavelengths must also have a limit?

I was looking at a video called How Hot Can It Get, and at 7:08, the host Michael states that if an object were to reach a temperature of $1.41 \times 10^{32}$ K (planck temperature), the ...
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graph relativistic degrees of freedom

I'm trying to graph the relativistic degrees of freedom, which should look like the figure And I am trying to guide me with this Phys.SE answer: Number $g(T)$ of relativistic degrees of freedom as a ...
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Blackbody radiation source

I have an instrument in the lab which needs to be calibrated. Ideally, it needs to be calibrated using a device that produces near-perfect blackbodies called a "blackbody calibration source". An ...
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Can radiators cool down using only back body radiation?

If you could produce a very thin tube, say less than the thickness of a human hair, designed to carry a hot liquid inside, the diameter is small so that the liquid would have a high surface area to ...
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What processes permit (presumed) emission by, but prohibit absorption of, IR photons by non-greenhouse gas molecules?

It is said that because of bond-structures / possible vibrational states, non-greenhouse-gases / homonuclear diatomics cannot absorb IR. Presumably there is no question that all gases, being matter, ...
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Principle of a pyrometer

How does a pyrometer work? Especially how do I ensure that the temperature of a single spot is measured (where the laser points at)? Here is a scheme of the structure but I guess it's not really ...
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Silica aerogel - reflectivity and absorption

Silica aerogels are 97 percent porous, meaning light moves through the material but the interconnecting nanolayers of silicon dioxide trap infrared radiation and greatly slow the conduction of heat. A ...
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How Can the Number of Degrees of Freedom per Frequency in the Rayleigh-Jean formula be Irrational, or are they not?

I've read that the Rayleigh-Jean formula for the energy density per frequency of blackbody radiation can be expressed as $u(ν,T) = nkT$ where k is Boltzmann's constant, T is the temperature, and n is ...
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Does the iron melt? Radiative-heat transfer

A molten metal (iron type A) is in vacuum (no air) at a constant temperature ($T_m$) equal to 1500 C (equal to its melting temperature) with emissivity $\epsilon_m$. At a distance of 10 cm there is a ...
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Do metallic meshes work in practice as thermal radiation shields?

The other day I was thinking about the wire mesh cover used in microwaves to keep the microwave radiation inside cooking your food. The idea is that the spacing of the mesh is much smaller than the ...