MadSci Network: Physics

Re: How do different coloured lights affect the temperature of objects?

Date: Sat Nov 28 19:13:11 1998
Posted By: Matthew Buynoski, Senior Member Technical Staff,Advanced Micro Devices
Area of science: Physics
ID: 911535164.Ph

In a word or two, I think you are dealing with absorbtion and reflection,
with heat capacity thrown in on the side.

Any object has the property that it reflects a portion of each color of 
light (Color is our perception. What is really varying is the wavelength of 
the light.) and absorbs the rest. Every material has different reflective
properties; this is due to the interaction of the internal electromagnetic
fields of the substance (from its atoms, and how its electrons are bonded)
and the photons (which are the carriers of electromagnetic forces) coming 
in. It is a little difficult to explain this further without getting into a 
lot of difficult technical jargon and solid state physics. I'll take
a shot at it, but if the next paragraph ends up confusing you rather than
providing enlightenment, just skip it and go on to the following paragraph.

Most materials exhibit what is called a bandgap, which means that no 
electrons in it can have energies between two levels (thus, a gap in the
energy spectrum). If an incoming photon has an energy greater than the
width of the bandgap, then it is likely to be absorbed. It will interact
with an electron and find it has enough energy to boost it up past the
bandgap into what is called the conduction band. Photons with lower 
energies tend to be far less likely to be absorbed and either pass
through (transparent) or are reflected.

If the particular material you are attempting to heat is reflective or 
transparent at the wavelength of the incoming light you are shining on it, 
it is unlikely to heat very much. If it is not reflective, then the energy 
of the photons is absorbed. Now, some may be reemitted at any wavelength 
longer than the incoming wavelength, and the rest will appear as heat.  How 
much is reemitted and how much is converted to heat is a function of the 
material. For example, I happen to have several fluorescent mineral 
specimens. When I shine ultraviolet (short wavelength) light on them, they
re-emit some of that energy as visible light (all visible light photons 
have longer wavelengths than ultraviolet photons) that I can then see. But
only a small part of the ultraviolet is converted to visible light.
Some materials have only a very faint fluorescence, but others can be
bright enough to actually see print. Some of the minerals, one feldspar in 
particular, are very reflective in the much so that it 
hurts your eyes to look at the specimen when the ultraviolet lamp is 
shining on it.

Now, once some of the incoming photons are converted to heat, how hot the
substance gets depends on another material property, its heat capacity. 
Some materials, like metals, take relatively little energy to heat to a
certain temperature. Others, like water, take a lot. What's going on here
is that in the low heat capacity materials (metals), it is easy to get
something (usually electrons) bouncing around real fast. In higher heat
capacity materials (water), the energy can be stored in myriad ways like
the vibration of bonds or rotation of molecules, that do not contribute to
what we sense as heat.

Almost everything discussed above is in the realm of what is known as
solid state physics. This subject is not an easy one, and I don't know of
a reference that isn't math-intensive above your likely level of 
mathematical ability (judging from your grade in school). The classic text 
is Kittel's "Solid State Physics", but I don't recommend that you go out 
and try this book as it is formidable. Rosenberg's book in the Oxford 
Physics series, title "The Solid State" is somewhat more approachable
but still beyond a high school level. I don't actually know of an easy
introduction to solid state physics, which perhaps reflects its level of 

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