Intro
In physics, a black body is a theoretical object that absorps áll possible frequencies or energy levels of EM waves, or similarly emits all of them. No radiation passes through it and none is reflected.
When emitting, how much electromagnetic radiation they give off only depends on their temperature.
Also the distribution of how much radiation occurs at which frequency is a very typical curve. See picture or click the "Black Body" layers for several curves of some discrete temperatures on top of eachother. The space between each curve is colored how we percieve that heated object. (Yes, a blue flame is hotter than a white flame, is hotter than a red flame.) Guess which temperature our sun has, by color!
A deeper understanding and reflection
Any matter that has a temperature radiates electromagnetic energy.
Heat is nothing more than the movement of molecules relative to each other or by themselves, such as vibration, rotation and torsion.(1) This heat energy gets released as EM Waves
And not only infra-red, though that is often the biggest part of it on earthly and daily objects and it's easy to deduct a temperature from them by measuring the IR radiation. Both are reasons why we tend to relate infrared with heat.
But also the visible and ultraviolet light we get, for example from the sun, but really from anything with a temperature, is mostly heat radiation.
(1)See more on this in the menu "Interaction with matter".
Also the opposite is true. When radiating EM waves to an object it will absorp it as heat.
Now, most matter exist of very specific molecules which have a myriad of ways to vibrate, but still only a certain limited number of ways. So it doesn't have all the possible energy levels of movement. That's why not always all frequencies will be absorped from external heat, or vice versa emited from its own heat.
Wikipedia link: https://en.wikipedia.org/wiki/Molecular_vibration
Red, white, blue? Temperature colors.
Black bodies below ±700K produce very little radiation at visible wavelengths and appear black. (We could see them via infrared camera's though.) Black bodies above this temperature however, start to produce radiation at visible wavelengths. The curve's peak starts to cross all the colors of the rainbow as the temperature increases, from red, via green ending up at blue . But we don't see these specific colors as a result. What we see, is the addition of all the colors the curve possesses at a certain temperature. You can't see green for example nor violet. The resulting colors go from red to orange to orangy white to white and than going to blue, with blue being the hottest.
Opposite to daily coloring labels (e.g. bath tub) blue is actually hottest and red coldest. That's probably because of fire and water, being hot and cold. In history our fires never reached a temperature hotter than red. Today we can see the white and blue clearly in very hot torches. Though, if we look carefully in wood fire, we can also see blue and some white, though it's mainly red/orange!
This range of colors is what is called "Temperature colors" or "Color Temperatures". They are more or less similar as the different colors seen at the horizon on a sunrise or sunset.
Interesting facts
- Temperature Sun: About 5780 kelvins.
Then why isn't the sun white in the sky? REad more on that in the chapter "Scattering". Here is a hint, the blue from the sky combined from the yellow of the sun, gives perfect white. - There is probably an evolutionary reason why our eyes see the sun as almost perfect white, ignoring the scattering. Would an alien evolved under a blue sun, precieve blue as white then?
- In the laboratory, the closest thing to black-body radiation is the radiation from a small hole entrance to a larger cavity. Any light entering the hole would have to reflect off the walls of the cavity multiple times before it escaped and is almost certain to be absorbed by the walls in the process, regardless of what they are made of
Calculating the black body curve was a major challenge in theoretical physics during the late nineteenth century. At that time, without quantum physics, it predicted infinite brightness at high frequencies (or, equivalently, short wavelengths), a ""ultraviolet catastrophe. As a result, the best-known theories at that time could not explain the observed spectrum of black-body radiation.
The problem was finally solved in 1900 by Max Planck. Without his knowledge, because he found the mathematics, but did not understand the physics of it, he was the inventor of quantum mechanics. - Even as the peak wavelength moves into the ultra-violet enough radiation continues to be emitted in the blue wavelengths that the body will continue to appear blue. It will never become invisible after that. Indeed, the radiation of visible light increases monotonically with temperature.
- In astronomy, stars are frequently regarded as black bodies, though this is often a poor approximation The spectrum of an incandescent bulb in a typical flashlight. Here, the filament temperature appears to be about 4600 kelvins due to a peak emittance of around 630 nanometers. However, the shape shows that the filament is not acting as a true black body.