How Light Transfers to Heat
What I would like to know, is HOW light absorption creates heat.
Light is a form of energy, and when it gets absorbed by an object/surface,
what does that mean? How is the energy transferred? Is it that the energy
from the light photon excites the bonds of the object it is being absorbed
by? Friction from bond vibration? What is actually happening at the
molecular/atomic level to cause heating of the object from light absorption?
All materials that are opaque or colored absorb various wavelengths of
visible "light". Even "transparent" materials like glass absorb
ultraviolet "light", and still others like water absorb infrared "light".
I use "quotes" because not all are visible to the human eye. These
wavelengths of light are absorbed by various components of the materials
and the absorbed light energy -- the extremes are a black object that
absorbs almost all the visible light and a white or reflective polished
object absorbs almost none. The absorbed radiation (light) is absorbed
further by various substances in the material. This absorption process
repeats and repeats until much of it ends up as infrared "light", that is
as heat -- that is vibrations of the molecules in the material. It is this
end product of these multiple absorptions that we sense as heat.
All the time this absorption is taking place, some of the radiation is being
returning to the surroundings.This process is pretty complicated and
different for different materials. But you are pretty much on target..
Higher energy "light" is converted to lower energy "light" (largely
infrared) that produces the sensation of heat.
Atoms absorb photons by moving an electron up an energy level -- this
means that the electron goes from a lower-energy state to a
higher-energy state (the higher energy state is called an 'excited'
state). The question you are asking is how this energy is converted
into thermal energy (or rather, how it is converted into vibrational
energy, otherwise known as temperature). The answer is that it happens
rather directly, through electrons. There are two ways to explain
this: the "technically rigorous" way involves complex quantum dynamic
mathematical equations (which would be far outside the scope of this
forum -- and outside of my expertise). The more simplified way is to
think of chemical bonds are just (shared) electrons, and vibrations in
chemical bonds are an example of varying energy in those electrons (as
well as the rest of the atom). So you can think of photon absorption
and chemical bond vibrations both as just different kinds of energy
fluctuations in electrons. Those fluctuations of energy can be
transferred to the nuclei and other electrons as well. This collection
of vibrations is what we experience as "temperature".
Hope this helps,
Consider what temperature represents. It is often described as the
average kinetic energy per molecule within a substance. When the
molecules bounce around harder and faster, the temperature of the
material registers as higher up. When very fast molecules crash into
very slow molecules, the fast molecules slow down while the slow
molecules speed up. This is what happens when a hot material makes
contact with a cold material.
When light energy is absorbed by the molecules of a material, the
molecules have more kinetic energy, they speed up. As this kinetic
energy spreads through the material, the material warms up.
Dr. Ken Mellendorf
Illinois Central College
Microscopically, on the scale of atoms, the temperature of an object
consists of how fast the atoms are jiggling around. Faster jiggling
means hotter temperature. When a photon hits a surface, it tends to
interact with the electrons in the atoms. The photon typically transfers
momentum to the electrons, which in turn transfer momentum to other
electrons. The net effect is to make everything jiggle faster -- i.e.
to raise the temperature.
Click here to return to the Physics Archives
Update: June 2012