physics question #1389



Russell Westhaver, a 33 year old male from Vancouver asks on April 12, 2003,

Q:

I've got a question about light. As I understand it, light is a particular form of energy--in order for something to produce light, energy of some kind has to be transmitted to or through that object which is later released as light--whether that's by rubbing two sticks together or through some sort of chemical reaction that produces energy and then light. Or something like that. My question: is there any way in which light might be produced by a reduction in energy--say by putting something in the freezer or cooling something? More generally, can energy be created through a reduction in temperature?

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the answer

Jess Brewer answered on April 13, 2003, A:

Yes indeed, light is usually generated as an object cools (as long as you use a broad definition of "light" that includes infrared etc.), but that is a rather unconventional way of looking at it. Any "blackbody" constantly emits a characteristic spectrum of electromagnetic radiation generated by random local thermal fluctuations. You can reduce this cooling mechanism by making the body less black (e.g. by wrapping it in aluminized Mylar or painting it white) but you can never turn it off completely. This is where "black ice" comes from: on a cold, clear, calm night the thermal radiation from a road (about 450 Watts per square metre) mostly escapes into outer space, and very little comes back down; this cools the road. If the ambient air temperature is just about freezing and the air is saturated with water vapour (as is often the case in the vicinity of Vancouver) then the road will drop below freezing and the water vapour will condense directly into an ice coating for the road. Citrus growers in Florida are aware that the leaves on their trees are pretty close to black, and will drop 10 degrees below the air temperature if left in still air on a clear night; so they use "wind machines" just to stir up the air and prevent this. At higher temperatures this radiative cooling just causes morning dew on the grass, a more harmless effect but the same in principle.

If you are looking to harness this radiation to generate power, you'd be better off to use a source with a higher surface temperature, such as the Sun at about 5000 Kelvin. Same effect, except the Sun has several ways of replenishing the energy it radiates away as blackbody radiation.

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