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Heat, Color, and Energy
Name: Akhtiar
Status: educator
Age: 30s
Location: N/A
Country: N/A
Date: 9/22/2004
Question:
When a metallic piece of iron is heated its colour becomes red,
then yellow and finally becomes white hot, if heated continuously. No
doubt heat is transferred to the piece of iron, which causes vibrational
motion in its molecules. My question is that what is the relationship
between colour and heat (or energy)? I know that energy is function of
frequency and colours depend upon frequency and hence on wave length. But
why colours are developed due to energy? or frequency or wave length?
Replies:
The color that is emitted by a body as a result of its temperature is,
ironically, called "black body radiation". The intensity distribution of
the various frequencies (wavelengths) of light is a function only of the
object's temperature, not its color nor the material (provided it does not
decompose at the particular temperature. People find that surprising. There
are many web sites and books that describe "black body radiation" and it is
a bit too long to present here. It was the radiation emitted by such bodies
that led Planck to assume that radiation was quantized and so it plays a
central role in the development of quantum theory. It is also a lesson in
how physicists think. Planck knew the experimental result. He also knew
hat
classical theory of radiation could not explain the observed result and
thought "backwards", "What do I have to assume about radiation to fit the
experimental result?" It is a fascinating detective story worth your time
to pursue. It is possible to obtain a 'flavor' for the story without going
into the mathematical details.
Vince Calder
Akhtiar,
The color of light is nothing more than an indication of energy level for
that light. A very hot object has more energy available for light than does
the same object when cold. Both objects radiate light waves, but the cold
object's waves are lower energy and lower frequency. In fact, a cold
object's light waves have such a low frequency that your eyes do not
respond. They are below the frequency of red light: infrared. As an
object heats up, the energy and frequency of the emitted light will
increase. Eventually, the frequency is high enough to stimulate your eyes:
red. As energy and frequency continue to rise, your eyes are stimulated to
register orange light. At very high temperatures, the light appears to be
white.
Ken Mellendorf
Math, Science, Engineering
Illinois Central College
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