Color and Temperature
Name: Julia R.
If you break up a light using a prism, would the different colors have different
temperatures? How does color relate to temperature?
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.
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
Dr. Ken Mellendorf
Illinois Central College
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Update: June 2012