I am confused about how hot surfaces can radiate light to their surroundings. When I shine a light on a surface the light turns to heat spontaneously, and when I leave that hot surface it radiates light spontaneously. To me this suggests that the process isn't driven by entropy increasing because it is two directional. Could anyone explain how this process complies with the second law of thermodynamics?
Answer
Heat, in the context of something "giving off heat" that we use in everyday conversation, is a term we use often to describe emission of a specific part of the electromagnetic spectrum (namely the infrared spectrum). As you start to pour more and more energy into an object, the electrons can get more and more excited (which is the process of absorbing photons), until they emit a photon to return to a lower energy state. As you get hotter the electron can jump up further and further until when it returns to the ground state it emits visible (or UV/x-ray) photons.
In terms of entropy and the second law, when shining the light on a surface, the electrochemical processes occurring in the flashlight results in a greater amount of entropy since the chemical concentrations of the battery cells increase in the number of micro-states. Similar would be true of whatever is driving the light emission from your source. When the energy is emitted from the surface, heat is flowing from a hot source to a colder one, increasing the number of potential states (multiplicity) and thus entropy of the entire system. Throughout the entire process entropy is having a net increase if you observe the entire system.
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