All Questions
Tagged with blackbody or thermal-radiation
1,722 questions
177
votes
1
answer
94k
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 ...
- 1,593
105
votes
6
answers
11k
views
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 ...
- 4,183
92
votes
7
answers
28k
views
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?
...
- 1,657
76
votes
8
answers
23k
views
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 ...
- 919
76
votes
5
answers
63k
views
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 ...
- 1,938
69
votes
8
answers
11k
views
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 ...
- 1,556
58
votes
6
answers
45k
views
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?
- 2,030
56
votes
3
answers
15k
views
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 ...
- 682
56
votes
4
answers
4k
views
Does anything in an incandescent bulb actually reach its color temperature (say 2700 K)?
This question is inspired by a question about oven lightbulbs over on the DIY stack. It spawned a lengthy comment discussion about whether an incandescent lightbulb with a color temperature of 2500 K ...
- 573
56
votes
2
answers
3k
views
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 ...
- 14.7k
52
votes
4
answers
18k
views
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?
- 1,005
50
votes
3
answers
8k
views
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 ...
- 1,088
49
votes
4
answers
16k
views
Could I survive at (or near) absolute zero with a very, very, very thick sweater?
Imagine I'm in an infinitely large vacuum and have a special apparatus built into my body that allows me to breath, eat, pee/poo, etc. and never age. The vacuum is similar to deep space and has no ...
- 589
44
votes
5
answers
13k
views
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 ...
user181531
44
votes
4
answers
13k
views
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 ...
- 563
41
votes
6
answers
4k
views
Why does canola oil heat up in the microwave?
Introduction: I read on Wikipedia's list of common misconceptions that microwaves work not by emitting the resonant frequency of water, but as a result of dielectric heating. As I understand it, this ...
- 619
40
votes
4
answers
26k
views
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 sky at night white?
I suppose it could be due to ...
- 775
39
votes
4
answers
7k
views
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 ...
- 2,557
38
votes
4
answers
11k
views
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 ...
- 591
37
votes
4
answers
6k
views
Why do we need insulation material between two walls?
Consider a slab made of two walls separated by air. Why do we need insulation material between the two walls. Air thermal conductivity is lower than most thermal conductivities of insulating material ...
- 1,970
36
votes
3
answers
16k
views
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 ...
- 481
35
votes
3
answers
6k
views
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 ...
- 353
35
votes
5
answers
7k
views
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 ...
- 8,260
33
votes
3
answers
5k
views
Force of photons from the Sun hitting a football field = weight of 1 dime?
I read, I think, some time ago that the "weight" of photons from the Sun hitting an area the size of a football field at noon on a sunny day would be about the "weight" of a dime?
...
- 447
33
votes
3
answers
2k
views
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 ...
- 4,331
33
votes
6
answers
15k
views
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 ...
- 2,784
31
votes
3
answers
4k
views
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 ...
- 425
31
votes
1
answer
69k
views
Why is black the best emitter?
Why are emitters colored black better emitters than other colors? Why is white a worse emitter?
- 425
30
votes
9
answers
8k
views
Why are good absorbers also good emitters?
I read that good absorbers are good emitters - hence a blackbody, that absorbs all kinds of radiation, also emits all kinds of radiation? I'm not able to get my head around this.
What does it mean to ...
- 1,931
29
votes
3
answers
7k
views
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 ...
- 621
28
votes
6
answers
5k
views
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 ...
- 1,841
28
votes
7
answers
8k
views
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 ...
- 1,918
28
votes
3
answers
15k
views
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 ...
- 293
27
votes
2
answers
9k
views
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 ...
- 441
27
votes
2
answers
6k
views
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 ...
- 672
26
votes
4
answers
18k
views
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 ...
- 1,772
25
votes
2
answers
4k
views
Estimate the Sun's temperature, based on the duration of sunrise
The full problem statement is:
Measured from the time when the first rays of sunshine appear above the horizon until
the moment when the sun is fully visible, sunrise lasts 2.1 minutes. Based on this
...
- 508
24
votes
9
answers
9k
views
If you were invisible, would you also be cold? [closed]
If you were invisible, would you also be cold? (Since light passes through you, so should thermal radiation.)
Additionally, I'd like to know if you were wearing invisible clothes, would they keep you ...
- 365
24
votes
3
answers
4k
views
Aren't all objects luminous in a sense?
In my physics class, I learned about "nonluminous objects" - these are objects which don't produce their own light. But, don't all objects emit light by black body radiation? So are all ...
24
votes
3
answers
10k
views
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 ...
- 5,452
24
votes
6
answers
2k
views
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:
...
- 976
23
votes
2
answers
1k
views
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....
- 21.2k
22
votes
3
answers
5k
views
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 ...
- 555
22
votes
2
answers
9k
views
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}{}} \...
- 530
22
votes
1
answer
3k
views
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 ...
- 1,399
22
votes
2
answers
7k
views
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 ...
- 9,444
22
votes
4
answers
517
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 ...
- 4,537
21
votes
4
answers
10k
views
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 ...
- 2,908