An LED (bulb) lights up when it is in a closed circuit with a battery. ‘Because electrons flow in the circuit’. But what is the mechanism that transforms this kinetic energy of electrons into light energy? Should I be asking a different question?
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For funzies, I’m going to try to talk in a little more detail, including some quantum mechanical effects.
The LED is made out of atoms arranged regularly in a crystal. Atoms have positively charged nuclei, and electrons around them. Each of these nuclei have a certain number of “slots” for an electron to fit. If the crystal were completely regular, these slots would all be full, and when that happens electrons can’t hop from one atom to another and electrical current won’t flow. (In metal wires the slots are partially full and electrons hop around freely.)
However, in an LED we replace some of the normal nuclei with different ones in a process called doping. The replacements have either extra electrons, or fewer. One side of the diode has extra, the other side fewer.
When you connect the LED to a power source, electrons come in on the extra side, flow over to the fewer side, and get attracted to a nucleus with an open slot. But they have too much energy to land in there-sort of like being in orbit: you have to lose kinetic energy in order to land. They get rid of that energy by emitting light.
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