# Tagged Questions

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

25 views

### Black body emission by a dielectric slab [on hold]

I need to calculate the radiation emitted by a rectangular dielectric slab (glass) at temperature $T_0$. The slab has a thickness $t$, sides $a,b$ and a dielectric constant $n(\lambda)$. How can I do ...
19 views

### Thermal radiation [on hold]

The emissivity of a hemi-sphere varies as: $$e=E\cos\theta$$ What is the ratio of emissive energy between the given sphere and a normal black body sphere.
2k views

### How can IR cameras see things colder than the camera?

I’ve read that only cold-blooded animals develop IR vision to spot warm prey because a mammal would blind himself with his own heat and not be able to see anything. But I’ve seen a FLIR camera, ...
35 views

### Is the universe inside some huge black body? [closed]

I've been reading about the black body radiation. A perfect black body can absorb all possible wavelengths- which can be put from zero to infinity. But that will depend on the size of the black body ...
49 views

### Could a human feel the black body radiation of another human standing behind her?

I've been thinking about infrared radiation and noticing more and more how the human skin seems actually pretty sensitive to it. You can easily feel a bonfire from several meters away, far away from ...
74 views

### A few question about the black body radiation

This is my understanding of the black body radiation: a black body is heated. The energy from the heat causes the electrons in the body to oscillate, so they will emit electromagnetic waves. If we ...
23 views

### Total spin of many photon thermal state [closed]

I need to find the total spin of a large spin 1/2 ensemble in thermal state: I'm trying to find a formula for $\operatorname{Tr}(S^2 \rho)$ where $\rho=\frac{1}{Z}e^{\beta S_z}$ is the thermal state ...
32 views

### Black body radiation and number of photons emitted

Usually the black body radiation (at a certain temperature $T$) is given by $$\rho ( \nu ) = \frac{8 \pi h \nu^3}{c^3 \left( e^{h \nu / (k_B T)} - 1 \right)}$$ This quantity $\rho ( \nu )$ should be ...
34 views

### How / why do infrared thermometers work?

How can reading the intensity of infrared radiation coming from an object let you calculate its temperature? Could any wavelength be used to measure temperature - why infrared, and not, for instance,...
24 views

### Problem on Emissivity and absorptivity

I have read the radiation chapter, where I have been introduced with the terms emissivity and absorptivity. emissivity tells about the ability to emit heat energy as thermal radiation compare to a ...
31 views

### Inequality of dimensions in Kirchhoff's law

I have read about kirchoff's law of radiation ,which tells that ratio of emissive power of a body to the absorptive power of a body is constant and it is equal to emissive power of black body So Qe/...
36 views

### Can an electromagnetic wave be split into multiple longer wavelength waves?

Is there any way to convert short wavelength (visible light/near infrared) electromagnetic waves into multiple long wavelength waves (microwave) of near equal total energy? Heating a blackbody with ...
57 views

### 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 ...
49 views

### Does fusion continue to happen as a star collapses into a black hole (beyond our view)?

I know that we can't actually see a black hole form, because of time dilation, and we would see a frozen image just before the final collapse past the event horizon, slowly redshifting to nothing... ...
40 views

I have some questions about blackbody radiation. In the classical calculation, we assume every mode has the same energy, then every mode should have the same probability, then why the curve of ...
65 views

### An ideal black - body at room temperature is thrown into a furnace. It is observed that? [closed]

An ideal black - body at room temperature is thrown into a furnace. It is observed that? (A) initially it is the darkest body and at later times the brightest. (B) it is the darkest body at all ...
35 views

### Moving heat problem: instant toast?

Toast-making usually takes a couple minutes using something like a conventional toaster oven to brown the surface of a piece of bread, making toast. However, this duration may be inconvenient. What ...
27 views

### Trouble in deriving the Rayleigh-Jeans formula from the steps shown on HyperPhysics site

Hi I am very new to this aspect of physics and 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 ...
42 views

### How does radiative cooling between the earth and space work?

On a chilly (but above freezing) temperatures on a clear night, you can freeze water outside because of radiative cooling. By what mechanism do warm bodies on earth actually lose heat to space? ...
30 views

### Heat loss by radiation reference

In our lab practice, we measured the rate of heat loss of a copper plate in atmospheric pressure and the rate of heat loss of the same plate in high vacuum ($\sim 10^5 torr$). So all I could find for ...
37 views

### understanding thermal radiation in a conductor, gas and insulator

Context: I was in my bliss of ignorance and happiness when I was taught that quantum mechanics was about nice discrete values of energies. Now I am introduced the idea of Fermi Energy in a block of ...
655 views

### Cooling down to absolute zero by radiation

Consider a system consisting of a gas, it is put in a container which is permits transmission of all kinds of electromagnetic waves. If this system is isolated and put in a perfect vacuum, and left ...
31 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 ...
106 views

### How do Black Bodies Absorb and Emit Radiation? [duplicate]

I have learnt how the gases of elements are able to absorb only certain wavelengths of EM radiation corresponding to the energy transitions between energy levels of orbitals. Furthermore, these ...
50 views

### Radiative Cooling of metal object

We have this experiment where a metal bar is heated and then we have to make a model for the cooling that occurs. We get numbers for how long it takes the metal bar to cool from 200 to 100 degrees ...
51 views

### Why do High Pressure Gases produce a Continuous Spectrum?

I am aware that low pressure/density gases produce an emission spectrum as there are specific energy transitions that the electrons can make, emitting certain frequencies of EM waves. However high ...
25 views

### Radiation-Glossy Black vs Matte White

Well, studying the chapter Heat I know that a matte black body radiates heat more than a glossy white one but can anyone give me an answer about what happens when its a glossy black body and a dull ...
105 views

### Is the visible light spectrum from “red-hot glass” at least close to Blackbody Radiation?

Briefly, for visibly transparent materials like glass, you can see through then even while they are glowing red hot. Most glasses have plenty of absorption in the IR, so there is plenty of absorption ...
38 views

### Finding the exponent of $\lambda$ in Wien's displacement law

I am reading this paper on a short history of the $T^4$ radiation law. In particular, on page 5, By assuming that the wavelength of radiation emitted by a molecule was a function only of its ...
57 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 ...
34 views

### Diminishing solar temperature and its effects on earth

This is a hypothetical question; considering both the earth and the sun as black bodies. If the temperature of the sun decreased N times, what would be the effect on the radiation intensity received ...
85 views

### If an object has a temperature, does it have to radiate?

I'm reading through a powerpoint presentation about Hawking Radiation (HR). They are explaining all of the reasons that built up to the postulate of HR, and one of the reasons is that if there is a ...
65 views

### Radiative equilibrium of a relativistic black body

A black body in empty space in the CMB rest frame should (discounting starlight) come to an equilibrium temperature of around 2.7 K. However, if the body is moving relative to the CMB frame, one side ...
86 views

### Is it possible to produce higher temperature than the light source with passive device only?

Suppose the light source is a black body with average temperature 2000K, and I want to use it to heat another black body to 2500K, via passive devices only. Here passive device means lens, mirror, ...
15 views

### Black body with radial radiation emission/absorbtion

Is there any model using some kind of material that can approach a spherical black body in a reduced scale?
30 views

### Power absorbed by an object inside a perfect blackbody cavity

Suppose you were inside a thick spherical shell of inner radius $R$, which was a perfect black body at some temperature T. What would be the power a sphere of radius $r$ would absorb located inside ...
36 views

### Is thermal transfer in a vacuum proportional to temperature difference?

I would have thought that heat dissipation was proportional to temperature difference, especially in a vacuum. Is this true?
39 views

### Black Body radiation (intensity in certain distance)

i need some help with this task: I have a black body radiator which is spherical and it has radius $r=0.56m$. It radiates with intensity $1.5\frac{kW}{m^2}$. I would like to know the intensity in ...
93 views

### How is Planck's radiation law related to $E=h\nu$

How did Planck conclude $E=h\nu$ from his radiation law?
60 views

### Emissivity and Final Temperature of a Black and White object

Objects can be categorized as blackbodies (emissivity $\epsilon = 1$), grey bodies (emissivity $\epsilon < 1$) and white bodies (emissivity $\epsilon = 0$). If we placed two objects (identical ...
58 views

### Carnot Engine- Cold Reservoir is a black body [closed]

My question: A satellite powered by a Carnot engine uses heat from a nuclear reactor at a fixed temperature T0. Heat is released into outer space via thermal radiation emitted by a set of fins at ...
46 views

### Characteristic room-temperature photon energy - is this nomogram wrong?

Reading this recent ars technica article on the James Webb telescope, something kept bothering me about the nomogram - shown below. The credit says it is from The Opensource Handbook of Nanoscience ...
34 views

### Synthetic Photometry - Calculating a colour index

I have a theoretical black body spectrum, described by plancks law. I also ave the bandpass sensitivity function for various filters. I would like to calculate a colour index from this information, so ...
426 views

### Why 8–15 µm is considered “thermal infrared” if typical room temperature kT is 48 µm?

According to Wikipedia: Long-wavelength infrared (8–15 µm, 20–37 THz, 83–155 meV): The "thermal imaging" region, in which sensors can obtain a completely passive image of objects only slightly ...
29 views

### When thermal IR gets reflected from an object, does it change its wavelength (frequency)

I'm working with thermal infrared (IR) cameras to detect human thermal radiation. I notice I can easily distinguish non-human objects throughout the camera's field of view, though all are at same room ...
61 views

### Planck's law - energy, frequency and temperature dependancy

Unlike radio waves, thermal radiations are emitted at a broad range of frequencies. So, at a given temperature, an object might be emitting a lot of frequencies. Is that correct? So ehen Planck's ...
26 views

### Isothermal expansion of a cavity containing electromagnetic waves?

The cavity is perfectly reflecting , and simulates a black body . Is it that during an isothermal expansion of a cavity containing electromagnetic waves the internal energy density is constant and ...
15 views

### Physics 3 recommended book with solutions [duplicate]

I'm new to this forum and wanted to ask for a recommendation for a book which has these subjects with problems and solutions:(if this is not a valid question please tell me) -Intro to Waves -Fourier ...