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How exactly does light transform into heat--for instance, when sunlight warms up a brick wall? I understand that electrons in the atoms in the wall absorb the light, but how does that absorbed sunlight turn into thermal energy?

It turns out that, as often happens in science, that there is more than one way to explain the same basic phenomenon.

Tom Zepf of the physics department at Creighton University in Omaha, Neb., notes that "Sunlight heats a material such as water or a brick primarily because the long wavelength, or infrared, portion of the sun's radiation resonates well with molecules in the material, thereby setting them into motion. So the energy transfer that causes the temperature of the substance to rise takes place at the molecular rather than the electronic level."

Scott M. Auerbach, a theoretical chemist at the University of Massachusetts at Amherst offers a more detailed answer:

"Light from the sun excites electrons in the atoms which constitute the brick wall. How does that electronic energy get converted to heat, you ask. The key is 'radiationless transitions.' Here's how it works: the atoms of the brick are perpetually vibrating. Some of those atoms vibrate sufficiently vigorously that their vibrational energy is roughly equal to the electronic energy (photons) absorbed from the sun--in essence, they are in resonance with the solar energy. Those atoms then make a quantum transition from 'electronically excited' to 'vibrationally excited,' meaning that the energy causes the whole atom to move. We feel that motion as "heat." The atoms which make the jump to vibrational excitation soon collide into neighboring atoms, dissipating their vibrational energy throughout the entire brick, making the brick hot throughout.

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