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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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