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Name: Paul
Status: educator
Grade: 9-12
Location: FL
Country: USA
Date: Summer 2009



Question:
Does resonance play a part in light to thermal conversion? For example a brick in sunlight heats to a certain temperature while a piece of metal typically is hotter (even if the metal is painted the same color as the brick) Is this because the metal is a better converter of light to heat due to atomic resonance of the materials being a closer match to the frequency of the sunlight striking it?


Replies:
Hi Paul,

The short answer to your question is no. You seem to have observed that a brick placed in the sun will not get as hot as a plate of metal painted the same color. There can be a number of explanations. One is that if you are determining temperature simply by "feel", the brick has much poorer thermal conductivity than metal, hence it will not conduct its heat to your fingers as quickly, and therefore will not feel as hot as metal that has high thermal conductivity.

A brick has much more of its surface area not facing the sun than a plate of metal, hence there is more area that can be cooled by ambient air (only one surface is receiving heat, whereas up to 5 are exposed to cooler air). Also, with its greater mass, a brick will take much longer to come up to temperature.

Energy from the sun is primarily in the infrared and visible wavelengths. In the case of a brick that is a dark color or a dark metal plate, nearly 100% of the visible wavelengths and the infrared is absorbed and that energy acts to heat the brick or metal plate. If the brick or metal plate were a true black body and absorbed 100% of the incoming energy, then every Watt that impinged on it would be converted into 1 Watt of heat. It is not possible, by "resonance" or any other method, for more energy to be imparted to the object, than it received in the first place, because that would violate conservation of energy principles. Any difference in temperature you detect must be caused by differences in the object's unexposed surface area, or other characteristics, or perhaps simply not waiting long enough for the brick to reach its final temperature.

Regards,
Bob Wilson


No, it is actually simpler than that. The resonance of the material is displayed simply by its color. It absorbs light of colors (frequencies) it resonates with.

The reason metals get so much hotter than ceramics is that the specific heat capacities of metals are much lower than the specific heat capacities of most other materials. So the same amount of energy added makes a metal hotter than a brick.

Richard E. Barrans Jr. Ph.D., M.Ed.
Department of Physics and Astronomy
University of Wyoming



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