Black Body and Radiation
What is the difference between a blackbody and an object
that is completely black?
Why do objects emit electromagnetic radiation when they are heated?
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
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.
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
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).
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Update: June 2012