: How does flowing electricity in a closed circuit lead to an LED lighting up?

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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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Things like light emitting diodes and tungsten filaments get excited by the flow of electrons through them and this excitement converts the electrical energy into light and heat energy.

Older incandescent lights worked by making part of the circuit a special kind of wire that electrons really don’t like to flow through. They scrape and bang and batter as they flow through that part of wire, which creates a kind of electrical friction. That friction heats up the wire so much that it glows, like a campfire cinder. That’s why those kinds of bulbs are really hot to the touch, they’re less light-making machines as they are heat-making machines that happen to glow as a side effect.

LEDs are trickier to explain correctly. You can imagine an LED like a huge cliff, with a high end and a low end. The power source stuffs electrons onto the high side and pulls them out of the low side, creating a situation where you have a ton of electrons crowded together at the top of the cliff and almost no electrons at the base of the cliff. The electrons don’t like being crowded, they’d prefer to be down at the bottom where they can get out of the crowd. Luckily, there’s a set of playground slides that will let them slide all the way down. But to ride the slide, they have to pay a toll. The price is 1 photon of light of a certain color. So electrons “pay up”, release a photon of light, and go down the slide. Those released photons make the LED glow in a specific color.

Basically, LEDs are, at their core, two different parts of a junction (hence a di-ode) where one side can only take electrons with a certain amount of energy. The electrons coming into the other side of the diode have too much energy, so the excess energy is released as light. (the common metaphor is a hole and the electron is too large to fit, so it has to shrink size).

Interestingly, this is the same principle as photo cells, only in reverse. If you run power into a solar panel, under the right circumstances, they will emit light.

https://www.youtube.com/watch?v=6WGKz2sUa0w

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.

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.