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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172
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1answer
86k views

Why does NASA use gold foil on equipment and gold-coated visors?

I've read several websites about equipment covered with gold foil and astronaut helmet visors are coated with gold. However, their explanations are devoid of almost all physics content. Can someone ...
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Explain it to me like I'm a physics grad: Greenhouse Effect

What is the mechanism by which increasing $\rm CO_2$ (or other greenhouse gases) ends up increasing the temperature at (near) the surface of the Earth? Mostly what I'm looking for is a big-picture ...
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Why do metals only glow red, yellow and white and not through the full range of the spectrum?

Why don't metals glow from red to yellow to green to blue etc.? Why only red, then yellow and then white? Shouldn't all wavelengths be emitted one by one as the temperature of the metal increases? ...
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If I stood next to a piece of metal heated to a million degrees, but in a perfect vacuum, would I feel hot?

A friend of mine told me that if you were to stand beside plate of metal that is millions of degrees hot, inside a 100% vacuum, you would not feel its heat. Is this true? I understand the reasoning ...
61
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5answers
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Is fire plasma?

Is Fire a Plasma? If not, what is it then? If yes why, don't we teach kids this basic example? UPDATE: I probably meant a regular commonplace fire of the usual temperature. That should simplify the ...
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Is the Hawking radiation of a charged black hole thermal?

Suppose you have a Schwarzschild black hole of mass $M$ and angular parameter $a = 0$ (no rotation). Question: is it possible to throw a charge $Q$ at a faster rate than it will be re-radiated? Will ...
53
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How does the Earth's center produce heat?

In my understanding, the center of the Earth is hot because of the weight of the its own matter being crushed in on itself because of gravity. We can use water to collect this heat from the Earth and ...
47
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8answers
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What are the various physical mechanisms for energy transfer to the photon during blackbody emission?

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 ...
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Why don't we use infrared light to heat food?

Why don't we use infrared (IR) or even the far IR just to heat food in a microwave oven instead of, of course, the conventional 2.45 GHz microwaves? Don't people call IR heat waves?
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4answers
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Cooling a satellite

Satellites are isolated systems, the only way for it to transfer body heat to outer space is thermal radiation. There are solar panels, so there is continuous energy flow to inner system. No airflow ...
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What would a blackbody sound like?

If a blackbody has a temperature such that its peak frequency was well within our audible range, for example $1\ \mathrm{kHz}$, what would that sound like if we used Planck's law to plot its spectral ...
40
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Why doesn't the brightness of a bulb change with time?

Household bulbs get alternating current, which means that the voltage of source and current in circuit keep changing with time, which implies that the power supply isn't constant. However, we don't ...
39
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Why most distribution curves are bell shaped? Is there any physical law that leads the curves to take that shape?

All the graphs shown below come from completely different fields of studies and still, they share a similar distribution pattern. Why most distribution curves Bell Shaped? Is there any physical law ...
38
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5answers
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Why do we feel heat from infrared light but not from shorter wavelengths?

My guess would be that light with a higher energy such as visible or UV would feel hotter, but this is not the case! Is this something to do with human senses or is there a physics explanation?
37
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4answers
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Why are solar panels kept tilted?

I have noticed that, in my country India, most of the solar panels are tilted southward at an angle of ${45}^{\circ} .$ Even on buildings with inverted V-shaped roofs, solar panels are still oriented ...
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3answers
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Emissitivity of copper - Why are copper heatsinks all shiny?

This wikipedia article states that emissivity of polished copper is 0.04, and emissivity of oxidized copper is 0.87 - more than 20x of the polished copper. So my question is - why are all copper ...
34
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3answers
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Why does the Earth cool at night time?

I do understand that open water and open ground cools by the means of convection — lower air takes the heat and goes up, where it cools. But why does the Earth lose energy and where does it go? Does ...
32
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4answers
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Why are stars white?

That is may be an easy question, but I am not a professional. The sun is a star, and when I look at the sun it is usually yellow. Why are stars in the night white? I suppose it is for the distance. ...
32
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The Pioneer anomaly finally explained?

Pioneer 10 & 11 are robotic space probes launched by the NASA in the early 1970's. After leaving our solar system, an unusual deceleration of both spacecrafts has been measured to be approximately ...
31
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4answers
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Is it possible to focus the radiation from a black body to make something hotter than that black body?

My previous question 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 ...
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Why don't gas flames radiate much heat directly, but metal objects heated by them do?

Gas barbeque manufacturers place metal bars, ceramic plates or lava rocks above the gas burner so that they radiate more heat towards the grill. Cooking directly over a single gas flame just wouldn't ...
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6answers
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Is a suit that hides a soldier's heat signature fundamentally possible?

I recently played "Crysis", a game where the protagonist wears a suit that allows the player to hide both himself and his heat signature. Then I watched Iron Man 3, where a kid suggests that Tony ...
28
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3answers
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How many X-rays does a light bulb emit?

I read somewhere that most things1 emits all kinds of radiation, just very few of some kinds. So that made me wondering whether there is a formula to calculate how many X-rays an 100W incandescent ...
26
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2answers
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How much additional mass does the Earth gain each day from solar radiation?

According to this answer, energy has some (minimal) mass associated with it. Therefore, when lots of energy hits the earth (such as solar radiation in a 24 hour period) shouldn't the earth gain some ...
26
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2answers
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Why is the color of lightning white or blue rather than nothing?

According to my knowledge, when clouds get charged, they produce sparks, which look like lightning to us. My question is, why does lightning have a color? It consists of electrons, and should not ...
26
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1answer
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Why is black the best emitter?

Why are emitters colored black better emitters than other colors? Why is white a worse emitter?
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Would wearing clothing that is black on the inside and white on the outside keep you cooler?

The Straight Dope ran an explanation of why nomads often wear black clothing - it absorbs heat better from the body. On the other hand, white clothing reflects sunlight better. Is it possible to get ...
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How can it be that the sun emits more than a black body?

As far as I know, a black body is an ideal emitter. So how can it be that a non-ideal emitter emits more radiation than a black body? This happens only in a very limited area at around 500nm, but it ...
24
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Black Body Golf Balls

The surface of a golf ball has about 35% more surface area (than a similar sphere) due to its dimples. So my question is simple, given identical radius, ideal black body material, and temperature: ...
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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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2answers
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Why aren't gas planets and stars fuzzy? [duplicate]

The edge of Jupiter looks very sharp. Even more bothersome, the edge of the sun looks sharp, aside from kind of a soup of particles floating above it. The sun's surface has an incredibly low density....
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476 views

Paradox?: What is the form of radiation experienced by a harmonically accelerated observer?

Theory predicts that uniform acceleration leads to experiencing thermal radiation (so called Fulling Davies Unruh radiation), associated with the appearance of an event horizon. For non uniform but ...
21
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What can we deduce by the fact that mirrors cannot get a ray hotter than Sun's surface?

I think it is quite counter-intuitive that some lenses or mirrors focusing sunlight to a single spot cannot produce a temperature higher than Sun's Surface. What could a scientist deduce from that ...
20
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3answers
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Why doesn't hot charcoal glow blue?

I was learning about black body radiation and the explanations given by Max Planck and Albert Einstein when a thought crossed my mind. When we heat an iron piece, its color changes gradually from ...
20
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3answers
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How do fusion reactors deal with blackbody radiation?

The plasma of the ITER reactor is planned to be at 150 million K. Using the Stefan-Boltzmann law, setting the surface area as $1000\,\mathrm{m}^2$ (the plasma volume is $840\,\mathrm{m}^{3}$ so this ...
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5answers
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Why does energy have to be emitted in quanta?

I was reading some popular science book and there was this sentence. It says that energy has to be emitted in discrete portions called quanta. Otherwise the whole energy in the universe would be ...
20
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7answers
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The strange thing about the maximum in Planck's law

I read that it makes a difference whether you calculate $\frac{dE(\lambda) }{d \lambda}=0$ or $\frac{dE(\omega)}{d \omega}=0$ in the sense that the maximum energy density with respect to the ...
20
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3answers
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What causes a black-body radiation curve to be continuous?

The ideal black-body radiation curve (unlike the quantized emission seen from atomic spectra), is continuous over all frequencies. Many objects approximate ideal blackbodies and have radiation curves ...
19
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4answers
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Why does thermal radiation only occur at infrared and visible frequencies?

The resources that I've checked out seem to say thermal radiation only occurs in the infrared and visible spectrum. For example my heat transfer textbook and the Wikipedia page on emissivity. In my ...
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5answers
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If perfect blackness violates the Uncertainty Principle, how isn't dark matter a violation?

In a recent tweet, Dr. Michio Kaku stated that perfect blackness would violate the Heisenberg Uncertainty Principle, i.e. every object must emit some radiation. I have two questions regarding this ...
19
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1answer
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Why don't Wien displacement law curves cross?

In the above image, the curves for different temperature dont intersect anywhere. Stefan-Boltzmann law and Wien displacement law dont preclude the intersection. Is it because if, for example, they ...
19
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2answers
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Why does my infrared thermometer say the sky is at -2 °C?

I just got myself an infrared thermometer. I wouldn't have been able to predict what temperature it would give me when pointing at the sky, but it turned out to be -2 °C the first time I measured, and ...
19
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4answers
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How can a star emit light if it is in Plasma state?

I understand that star is in Plasma state (all nucleus and electrons are not bound to each other and moving around freely) Photon is emitted when an excited electron moves back to lower orbit. So in a ...
19
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3answers
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Why would different metals glow red at different temperatures?

According to everything I've been taught about incandescence and black-body radiation, and some quick Googling to confirm I'm not crazy, just about everything, regardless of composition, should start ...
19
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5answers
985 views

Can you ride Hawking radiation away from a black hole?

Suppose you were held by a very strong rope at constant Schwarzchild coordinate $r = 2M (1 + \epsilon)$ just above the event horizon of a Schwarzchild black hole of mass $M$. You would feel a proper ...
18
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3answers
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Why do lightbulbs continue to glow after the light is turned off?

I've noticed that whenever I turn the lamp off in my room at night, the lightbulb seems to continue to glow for a minute or so after that. It's not bright though; the only way I even notice it is if ...
18
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5answers
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Entropy of radiation emitted into space

In several papers I see something equivalent to the following expression for the entropy of radiation given by an astronomical object such as the Sun (assuming the object can be approximated as a ...
17
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2answers
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How to calculate how many photons are in the universe?

The "universe" is a sphere with a radius of $10^{25}$ m the medium temperature is 3K, how many photons there are in the universe? $$n_\gamma = \int_{0}^{\infty} \frac{8h\pi\nu^3} {{c^3}{}} \...

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