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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You’re asking the right question. It’s a really good question. It’s going to be really hard to make this an ELI5, because the conversion of electrical energy into light energy isn’t something humans can observe directly.
Electricity, or electric current, is the movement of electrical energy through (or around, but let’s not get nit-picky here) a conductor. Some materials are highly conductive, others are only semiconductive. We can tweak the conducting properties of a semiconductor by “doping” it with impurities, creating regions that are more favorable or less favorable to free electrons. Between the regions are *semiconductor junctions*, across which electrons can only flow in one direction. When an electric current is applied to such a material, a free electron from one region combines with an “electron hole” from another region, emitting a photon from the *depletion region* that forms between them. I can’t explain how. It’s in the realm of quantum physics.
If you really want to understand it beyond just assigning fancy names to the phenomena, these are the topics you’ll want to research:
How does an LED turn electricity into light? **Electroluminescence**.
How does electroluminescence work? **Radiative recombination** of charge carriers (e.g. electrons and electron holes in a semiconductor) produces a photon.
How does recombination produce a photon? When an electron crosses a **band gap** in a material from a higher energy level to a lower one, the excess energy is released as a photon. (We call this **spontaneous emission**.)
How does spontaneous emission work? Quantum electrodynamics.
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