- #1
fog37
- 1,568
- 108
Dear Forum,
An object, like a metal, that is highly reflecting (white body) is very different from a very emissive object (black body).
An object that closely behaves like a black body absorbs all the incident radiation. But it also "emit" all the absorbed radiation (over a range of wavelengths). Emission implies that the object has first absorbed the radiation. An ideal white body reflects instead all the incident radiation without absorbing it. Is this correct? Reflection does not imply that the process of absorption takes place.
At the end of the day, it seems that if 200J of energy are incident on a perfect black body or a perfect white body, all that energy will eventually be returned to the environment. That means that an object that does not have an internal source of energy eventually gives back all the energy it receives (with one mechanism or the other), correct?
I am sure there are some differences. MAybe in the case of a perfectly reflecting object all the reflected energy has exactly the same spectrum as the incident energy. Maybe that does not happen in the case of a black body. However, I think I read that the absorption spectrum of a black body has the same shape as the emission spectrum of the blackbody...
So why does a dark object feel hotter than a reflective object? Via conduction, we receive energy from the hot dark object. But we should receive energy also from the reflective object and feel it hot...why not? Maybe the feeling of hot and cold only depends on touching (conduction) and reflection does not involve conduction. Reflection is only about radiation.
To keep a liquid hot inside a bottle, should the bottle inside walls be dark or light in color? Eventually, all the heat emitted by the liquid will be either reflected or emitted back into the liquid...
thanks,
fog37
An object, like a metal, that is highly reflecting (white body) is very different from a very emissive object (black body).
An object that closely behaves like a black body absorbs all the incident radiation. But it also "emit" all the absorbed radiation (over a range of wavelengths). Emission implies that the object has first absorbed the radiation. An ideal white body reflects instead all the incident radiation without absorbing it. Is this correct? Reflection does not imply that the process of absorption takes place.
At the end of the day, it seems that if 200J of energy are incident on a perfect black body or a perfect white body, all that energy will eventually be returned to the environment. That means that an object that does not have an internal source of energy eventually gives back all the energy it receives (with one mechanism or the other), correct?
I am sure there are some differences. MAybe in the case of a perfectly reflecting object all the reflected energy has exactly the same spectrum as the incident energy. Maybe that does not happen in the case of a black body. However, I think I read that the absorption spectrum of a black body has the same shape as the emission spectrum of the blackbody...
So why does a dark object feel hotter than a reflective object? Via conduction, we receive energy from the hot dark object. But we should receive energy also from the reflective object and feel it hot...why not? Maybe the feeling of hot and cold only depends on touching (conduction) and reflection does not involve conduction. Reflection is only about radiation.
To keep a liquid hot inside a bottle, should the bottle inside walls be dark or light in color? Eventually, all the heat emitted by the liquid will be either reflected or emitted back into the liquid...
thanks,
fog37