Thermal Radiation Dependance on Temperature
All bodies with a temperature above absolute zero emit
thermal radiation. The type of radiation emitted depends only on
temperature of the object emitting the radiation.
why does it only depend on only temperature?
You have to be careful not to over generalize the statement. Not all
radiation emitted by a body is necessarily dependent only on the
temperature. Various gaseous discharge tubes can produce strong
monochromatic light (and other electromagnetic frequencies -- A low
pressure mercury lamp for example produces intense ultraviolet radiation at
254 nm.). The radiation you are referring to is called "black body
radiation" -- an unfortunate historical name that is a bit confusing
because the body need not be "black". The "reason" it depends upon only the
temperature is that it is produced by the vibrations of oscillating
electrical charges. What these oscillating charges are bound to --atoms,
molecules, particles -- is not relevant. Richard Feynman's "Lectures on
Physics" Vol I provides more details.
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Update: June 2012