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Name:  luli
Status:  student
Age:  19
Location: N/A
Country: N/A
Date: 2000-2001


Question:
What is the difference between a blackbody and an object that is completely black? Why do objects emit electromagnetic radiation when they are heated?


Replies:
I would like that you give me the answer. Thank you very much.

A blackbody is an object that absorbs ALL radiation that hits it. A blackbody reflects NOTHING, not visible light, not ultraviolet, not infrared, not even radio waves. An object painted black absorbs visible light, but not necessarily anything else.

Atoms have specific energy levels for their electrons. When energy is absorbed by an atom, an electron goes to a higher energy level. If there is nothing to keep the electron at that higher energy level, it will drop back down. This causes the extra energy to be emitted at electromagnetic radiation. When material is heated, energy is added to the atoms. The electrons go to higher energy levels, then drop back down and release the energy. Each material has its own radiation energies. This is why different materials glow with different colors when heated.

Kenneth Mellendorf


There is no difference between a blackbody and an object that is "completely black". The snag is that "completely black" implies more than that we perceive it as black. It must absorb not only all visible light but light at every other wavelength as well -- from x-rays to microwaves, radio waves, and beyond.

All objects are always emitting electromagnetic radiation, not just when they are heated. But they are also absorbing radiation. When they are at equilibrium with their environment they will absorb just as much radiation as they emit and their temperature is stable. A hot object will emit more radiation (energy) than it absorbs and thus will cool until it reaches the same temperature as its environment.

For radio, electromagnetic radiation is produced when electrical charges oscillate in a circuit. On a molecular scale a similar process occurs when an electron moves from one energy level to another or molecular vibrations or rotations change (note that a change must occur because energy is being emitted from the molecule).

Greg Bradburn



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