Resonance and Light to Thermal Conversion
Date: Summer 2009
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?
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.
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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Update: June 2012