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Name: Stephani
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Question:
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?


Replies:
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.

Vince Calder


Hi Stephani,

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,

Burr Zimmerman


Stephani,

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
Physics Instructor
Illinois Central College


Hi Stephani,

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.

Douglas Stanford



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