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Name: Julia R.
Status: student
Age: 17
Location: N/A
Country: N/A
Date: 9/30/2003


Question:
If you break up a light using a prism, would the different colors have different temperatures? How does color relate to temperature?


Replies:
This question is more complicated than it appears at first. Normally one associates temperature with infrared radiation. So if you ran a small mercury thermometer across the light spectrum there would be no temperature increase until the thermometer was positioned at some wavelengths longer than red. For all practical purposes mercury reflects all visible light and so it would not absorb any electromagnetic radiation in the visible part of the spectrum.

If a thermocouple, coated black to absorb all incident radiation, there would be an increase in the temperature across the visible spectrum as well. The temperature at any given position would depend upon the intensity of the incident radiation at that wavelength. If the source were a laser (very narrow wavelength) an increase in temperature would only be observed at the position where that single wavelength struck the light-absorbing thermocouple. If the source were an incandescent filament the distribution of intensity of the radiation would follow Planck's black body formula and the light-absorbing thermocouple would respond accordingly. The actual formula is rather messy, but it depends upon the wavelength of the light (the position of the thermocouple) and the absolute temperature of the radiant source.

Vince Calder


Julia,

Color of light indicates energy, but not temperature. The energy in a beam of light can be absorbed by matter as heat energy. This heat energy can then cause the molecules to vibrate faster. This vibration is temperature. The more the molecules vibrate, the higher the temperature. Color also determines whether light is absorbed or reflected. A red surface reflects red light but absorbs blue light. Shining red light on a red surface will not heat the surface. The energy light just bounces off. Shining blue light on a red surface will heat the surface. The light is absorbed by the surface,causing temperature to increase.

Dr. Ken Mellendorf
Physics Professor
Illinois Central College



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