I have a question regarding electric circuits, specifically when it comes to converting electromagnetic energy into light in a light bulb (tungsten)filament. How can an electron in a tungsten atom of the filament release infrared light photons once it comes back to its original state, after being excited by kinetic energy from the electric current. Where do the infrared light photons come from?

“When heat is transferred to an atom, it starts to vibrate more quickly. This vibration is a form of kinetic energy. Some of the kinetic energy is transferred to the electrons around the nucleus. This makes them “jump” from their usual shell into a shell that is further away from the nucleus. When an atom’s electrons move out of place like this, it is said to be in an excited state. This excited state is very unstable, and the electron quickly falls back down to its normal shell, and ground state. When this happens, the electron releases the extra energy it had gained in the form of infrared light photons.These photons are invisible to the human eye.”

From https://letstalkscience.ca/educational-resources/stem-in-context/what-causes-hot-things-glow

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    \$\begingroup\$ Comments are not for extended discussion; this conversation has been moved to chat. \$\endgroup\$
    – Voltage Spike
    Nov 25, 2020 at 17:47
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    \$\begingroup\$ To be clear, this question has nothing to do with electronics, and everything to do with the basic physics of what happens when matter gets hot. As such it is off topic under EESE rules and does not belong here. If there is an appropriate place in the SE system, that would be physics SE, not EESE. Debates on what EESE rules should be belong on meta, not the questions themselves. \$\endgroup\$ Nov 25, 2020 at 17:48
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    \$\begingroup\$ I like the way viewpoints contrary to yours have been hidden in chat, while you erge yourself at super-partes authority able to decree what is an what is not on topic, when you do not even realize that fields are but forces per unit charge and clearly believe that electronics is only about putting circuits together, treating components as black boxes. This is a question about the energy balance in a resistor, and is about electronics like other question about how capacitors and inductors work. \$\endgroup\$ Nov 25, 2020 at 20:41
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    \$\begingroup\$ I'm not in favor of the previous commentary having been removed, but facts are facts. This is about the thermal behavior of matter irrespective of the source of heating (eg deleted blowtorch comment), and therefore not an EM issue and as such the question has nothing to do with "electronics" and is off topic under actual EESE rules. If you don't see that, then you don't understand the physics involved here \$\endgroup\$ Nov 26, 2020 at 0:15
  • \$\begingroup\$ @ChrisStratton That is your interpretation. What I see is "a question regarding electric circuits, specifically when it comes to converting electromagnetic energy into light in a light bulb (tungsten)filament.[...] Where do the infrared light photons come from?". Once it has been made clear that photons are quanta of energy, the question becomes 'where does the energy released as IR light from a resistor come from?' And this is not only 100% electronics, in the sense described in the hidden comments, but very specific to the resistor case. \$\endgroup\$ Nov 27, 2020 at 1:51

2 Answers 2


I do not believe the active users of Physics SE are patient enough to write or read explanations that can be understood by most of us - the practical electricians. I write an answer here.

Elementary concepts such as "radiation is emitted when electron returns to lower energy orbit" unfortunately explain too loosely what happens in solid materials.

At first solid conductive material doesn't have some sparse allowed electron orbits. When interatom distance is small enough atoms disturb electrons in other atoms, the number of allowed orbits is vastly increased. A part of electrons move in so complex orbits and long distances that they are practically free in the emptiness between the atoms when compared to electrons in tighter lower energy orbits.

Atoms attract indirectly also each other when many atoms attract the same electron. This keeps the material solid - the distribution of the electrons between atoms happens to be a total energy minimum.

The number and variety of the possible electron orbits is so huge that only statistical distribution calculations are possible.

So, what makes electrons to jump from lowest energy orbits to the numerous free upper energy orbits which are possible in solid materials? Thermal excitation, they say. What's that? It's the mechanical thermal motion they say - random vibrations which cause thumps also to electrons.

But there's no such thing as mechanical thump. Electron can change it's course to higher energy orbit ONLY by absorbing a photon and to a lower energy orbit by emitting a photon. When do these things happen depends on are free orbits available, is there radiation available to be absorbed and the life statistics. Electron stay in certain orbit a random time which cannot be determined. Only statistical analyses are possible. That's one basic facts of the quantum physics.

Atoms can vibrate around their equilibrium positions - that's what the heat is. But all inter-atom thumps are relayed by the common electrons. Together with the previous paragraph that means the material is full of radiation - photons. Physicists handle photons as gas which obeys certain statistics. It's not the same statistics as particles obey because photons born and vanish all the time due the state transitions of the electrons. But their statistical nature is theoretically derived and it explains at least one phenomenon very well: All the time some photons escape out of the surfacea of solid materials. Those photons you already know. It's the thermal radiation, mostly at infrared wavelength range in temperatures that we humans can stand, but also visible light if we warm up materials at least several hundred degrees above our room temperature.

Electric current is a way to generate heat. That's because moving electrons contain extra energy. Besides it they are part of the electron cloud in the solid and obey its statistics so they emit photons and finally cause thumps to atoms. Due that energy loss the drifting motion of the electric current electrons in solids is very slow, only in vacuum electrons can get cosmic average speeds. But the number of drifting able electrons is especially in metals so high that substantial currents are possible.


Photons are quanta of energy that can behave like particles. But they are not 'stored' in the system in the form of 'particles'. They are the result of the interaction of your system with other systems or the environment.

So, you should ask yourself: if I have energy coming out of my system (one electron, or one atom, or an ensemble of atoms like a solid piece of tungsten), where do this energy come from? The (most likely) answer is that it comes from your system: you had an amount Ei before the interaction, you have an amount Ef after the interaction.

When you do the bookkeeping with the simplest systems you find out that the energy released is quantized in chunks that we call photons.

Sorry, but I am not going to waste my time in writing a deeper answer to a question that will be closed, but let me add this to address the core of your question: the photon energy comes from the electromagnetic field of the circuit composed by the battery, the ideally perfectly conducting wires and the resistor (the tungsten filament in the light bulb). Assuming a steady state current is flowing, if you compute the electric and magnetic field around the circuit you will find a configuration like this

Poynting vector in DC resistor circuit
source: Ian M. Sefton, "Understanding Electricity and Circuits: What the Text Books Don’t Tell You", Science Teachers’ Workshop 2002

where the Poynting vector goes from the battery to the resistor, guided by the conductors. This is the flow of energy shown by classical electromagnetism. What is not shown is the energy flowing out of the resistor in the form of infrared photons.

If we consider photons as quanta of energy, the question becomes where does the energy released as IR light from a resistor come from? Of course it comes from the battery (where could it come from, a nuclear explosion?) but once the energy has flown from the battery to the resistor as predicted by Poynting, explaining the energy conversion mechanism taking place in a resistor is all but simple.

The electron-phonon (yes phoNon, NOT phoTon) interaction by which the electrons accelerated by the field transfer energy and momentum to the lattice is inherently linked to the electrical conduction process and is crucial to correctly explain the dependency of the material's resistivity on temperature. (It also explains quantitatively the relationship between electric and thermal conductivity - the Wiedemann-Franz law.) Then, IR energy can be explained as a result of the semiclassical conversion of vibrational lattice energy into electromagnetic radiation via (mostly) electric dipole radiation. But on EESE, for some reason, this has nothing to do with electronics.

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    \$\begingroup\$ What you are missing is that this is governed by the temperature of the material alone, not the electrical input. No electric field is required for this to happen; the material merely needs to be "hot" for some reason - friction, blowtorch, nuclear reaction... it's all the same, only the temperature matters. \$\endgroup\$ Nov 26, 2020 at 0:34
  • \$\begingroup\$ And what you fail to see is that the temperature T at which the resistor arrive depends on how the electromagnetic field energy is transferred to the lattice. Because, at least in first approximation (the flawed Drude model), Joule heating and electrical resistivity are two sides of the same coin. Or are you arguing that the heat coming out from a resistor comes from nuclear explosions? \$\endgroup\$ Nov 27, 2020 at 1:52

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