Thermal electromagnetic radiation by an opaque and non-reflective body, also called a black body is called black body radiation. The black bodies absorb all the radiation that falls on them.
Black Body Radiation and Black Holes
One of the best examples of a black body is a black hole. They can be said perfect black bodies as they absorb all the radiation that falls on them. As you may know, they don’t even spare light from their trap. You may learn more about them from one of our previous articles Check Articles where we have even explained about temperature of black holes. Black Holes are already proven by the First Image of Black Hole.
Do Black Bodies Emit?
The black bodies absorb everything that doesn’t necessarily imply that they don’t emit either. They do emit radiation. For instance, black holes emit black-body radiation which is known as Hawking radiation. So, a black body emits thermal radiation as per their temperature. Almost every object in nature emits some kind of radiation according to its temperature. Let’s take colder objects for instance. Sometimes we think they are really cold, so they must not be emitting anything. But that’s a wrong notion. Cold bodies emit waves like radio waves or microwaves with high wavelength or low frequency. The exact opposite happens in the case of hot bodies as they emit ultraviolet or visible light that has a low wavelength and high frequencies. Similarly, black bodies emit radiation that is called thermal radiation or cavity radiation or temperature radiation. This emission occurs in a continuous spectrum according to their temperature.
Do Humans Emit Radiation Too?
Well, that’s a great question. And the answer is yes. Yes, you also emit radiation all the time. But we can’t observe that as that emission falls in the category of the infrared spectrum of electromagnetic radiation. Well, why do we emit infrared? The answer is really hot bodies like stars behave almost like black bodies. Stars appear really different from each other in colour as all of them emit different types of radiation. A really hot star like our sun emits radiation in the yellow or green part of the spectrum. A comparatively cooler star like a red star emits in the red wavelength of the spectrum. Sometimes even the spectrum can be in the region of blue or ultraviolet or gamma-ray radiation for hotter bodies. So, in comparison to those stars, planets and humans are comparatively cooler, hence they emit infrared.
But How Can A Black Body Emit If It Were A Perfect Absorber?
This is one of the most asked questions while studying black bodies, and actually there’s nothing wrong with the question too. Curiosity will automatically arise as to how can it emit when it absorbs everything and reflects nothing. Well, one has to first clarify oneself that it absorbs everything falls on it doesn’t mean it won’t emit anything. The answer lies in the definition itself, that is, the black bodies remain in thermal equilibrium with its surrounding. Thermal equilibrium means the rate of absorption equals the rate of radiation. A black body absorbs a lot of energy, so in order to equal the rate of absorption to the rate of emission, it must emit that much to keep the system in thermal equilibrium. Hence, this is the answer to the above question. It doesn’t reflect the radiation it absorbs, instead, it emits. The two words stylized as italics have totally different meanings, so don’t confuse them with each other.
Do Perfect Black Bodies Exist?
No, Perfect black bodies don’t exist in nature. The best examples of black bodies are graphite and lamp black. The emission of black bodies doesn’t depend upon any direction, so it’s called a diffused emitter. The emissivity of a perfectly black body is one. If you are looking for a visual understanding of what a black body is or what black body radiation is, go check this link
Author: Som Abhisek.
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