Photons -- How are they made and other questions

If I strike a match, photons are created. How? Are photons created the same way by a star? What are photons made of? Since photons have no rest mass, do they instantly move at the speed of light (no acceleration)? What exactly is rest mass?

And from this thread:

Chronos said:

How does this work?

Picture an atom with electron orbiting it. The electrons must orbit in rigidly defined distances from the atom. So, say there are three orbital shells for an atom (different atoms have a different number of electrons and orbits available to them) An electron can orbit in shell 1 or shell 2 but it can never orbit between shell 1 and 2. Each of these shells represents an energy state…the higher the shell the greater the energy the electron has. Now, if an electron, for an number of possible reasons, pops into a lower shell it must shed the energy it possessed from the higher shell. It does this by emitting a photon.

Note that I said earlier that each type of atom (e.g. hydrogen, iron, uranium, etc.) has distinct orbits for its electrons. As a result each atom will give off different wavelengths of light (or differing energy in its photons if you prefer) than other atoms. This is how astronomers can determine the chemical composition of stars. By studying the light we receive from a given star and knowing the wavelengths given off by different atoms you know what a star is made of to make the light we see. Because each electron must be in a defined orbit the light it releases will always be the same.

In most cases atoms don’t emit light. They are stable and the electrons generally don’t pop into different orbits. What usually happens is you add energy to the atom causing an electron to jump up an orbit. However, the electron doesn’t want to stay there so it drops back down and releases a photon. In the case of the match the chemical reaction produces heat which excites nearby atoms causing the electrons to jump up…when they drop back down a photon pops out and we see light from the match.

Photons always travel at light speed…there is no acceleration.

As to the part Chronos posted he’ll have to answer that.

Do electrons orbit the nucleus? In this thread I was told they don’t.

So photons are contained in electrons? Does the electron loss mass and/or energy when it releases the photon?

Electrons dont actually orbit, though we refer to the areas where there is a certain probability of finding an electron an ‘orbital’.

As whack-a mole implied, the ‘release’ of a photon is the way an electron gives off energy as it moves to a state of less energy. A bit like when you drop a book on the floor, it makes a noise - well, not really but I cant think of a better analogy at the moment.

Ok, this could be totally wrong but here I go anyway. When the electron comes back to a lower “orbit” it releases energy because it gained energy going to a higher “orbit”? Or it releases energy because it gained energy by coming back down in “orbit”? Is the total energy of the electorn the same a before the whole process started?

Conceptually, your explanation works… but you’re mixing models, one of which is the Bohr model (which was shown to be incorrect shortly after it was formulated, but it makes the answer to this question easier, so people still insist on using it). Again, it’s a pet peeve of mine, so I have to point it out.

Electrons do NOT orbit the nucleus at a fixed distance. Electrons exist in probability clouds, and can technically be found anywhere from the nucleus to infinite distance from the nucleus, they just tend to be in clouds of certain sizes and shapes. When they absorb energy, they go to a higher energy orbital (different from an orbit–an orbital describes the approximate size and shape of the probability cloud). When they drop back to a lower-energy orbital, they emit a photon.

As for what a photon is made of… well, it’s made of a photon. Photons cannot be divided into smaller pieces (we’re pretty sure that’s true, even though physicists have said that about many things, and still proven that those “indivisible” things were made up of smaller things–but photons are probably REALLY indivisible).

As for where it comes from… I can’t answer that. Hopefully another doper can get this better than I can (I’m more on the chemistry side, and have to admit this is something that I don’t completely understand in physics). I know energy can change forms, and basically what’s happening is the energy of the high-energy orbital is changing to light energy in the form of a photon. I don’t know if the high-energy electron can be said to technically “contain” a photon, but that’s the only way I can conceptually wrap my brain around it right now.

Sorry, I should have quoted the explanation I was answering (it was Whack-a-Mole’s).

Kong: It releases energy because it’s losing energy as it goes to a lower-energy orbital (in order to go from high-energy to low-energy, it needs to release energy in the form of a photon).

Why does the electron go back to a lower “orbit”? Why not keep the energy and maintain a higher "orbit?

The electron ‘wants’ to move to a lower energy orbital, because like everything in the universe, it wants to be less energetic - it wants to increase entropy by sending out an electron.

If Major Kong stood up all day, Major Kong would ultimately want to lie down after a while, wouldnt you.

So why doesn’t the electon just release all its energy in the form of photons from the start?

Did you mean:

it wants to increase entropy by sending out a photon.


It does, usually. Some external stimulus (energy input) causes an electron to jump to a higher orbit. The electron quickly drops back to the original orbit, releasing a photon.

So why do we have so many atoms left? sorry if this sounds like a dumb question.

Yeah I did mean photon. Ta.

You mean release its energy before it jumps to a higher energy level?

If it is in a relaxed state, the electron is already as ‘down’ as far as it can go. Other lower energy orbitals (if any) would already be occupied by other electrons.

Not sure if I get this question.

Most of the time, the electrons are in the lower energy orbitals. They only spend brief periods of time in an excited state, after which time they drop down to an available lowere energy orbital.

I don’t understand this. Why are there hydrogen atoms? There is only one electron, so why doesn’t this electron just release all it energy as photons? An in other atoms. Why doesn’t the electron closest to the nucleus release all its energy, then the next electron and so on. I am sure there is answer to this, I just don’t know it.

But why do they come back down? Why are they at the “height” they are to start with?

well if you consider photon to be light, and light to be electromagnetic wave. then a photon is nothing more than a propagating disturbance ( wave ) in an electromagnetic field. you throw a rock into water and waves are created, you dont ask yourself the question where did the waves come from :slight_smile:

as for atom. the lowest orbits are the stable ones, the higher ones are unstable. the electron will only be raised to a higher orbit when energy is applied to the atom, such as for example another photon striking the atom. it will fall back to a lower orbit spontaneously.

because an electron is an electric charge, when it moves it generates electromagnetic radiation. think of an antenna - how does it radiate electromagnetic waves ? it does so because the electrons inside it are moving with the current.

so when electron falls to a lower orbit it emits a photon.

take the hydrogen atom as an example, the electron doesnt ‘drop’ any furthe towards the nucleus because the nucleus exert a repulsive force on the electron. The orbital thet the electron inhabits (which is really the electron itself - dont worry to much about that) is at a distance from the nucleus where there is a minumum energy - a balance of the repulsive and attractive forces.

they “come back down” because they want to reach miniiimum energy

To make sure this is clearly answered… an electron “closest to the nucleus” (in the lowest-energy orbital) can’t release more energy to move somewhere else–as antechinus mentioned, it’s already at the point where attraction and repulsion are balanced.

So, why do the higher-energy electrons not collapse into that lowest-energy orbital? Each orbital can hold 2 electrons (each with different spin, but we won’t get into that). If the orbital is already full, it would be higher-energy to try to force a third electron in there than to put the third electron in an empty orbital. So, although the electrons in the “outermost” orbitals are higher-energy than the electrons close to the nucleus, those high-energy electrons don’t have anywhere to go to have lower energy.

As for why electrons are at higher energy to begin with… they’ve already absorbed a photon, which (briefly) knocks them to a higher orbital. I’m pretty sure there are other ways for them to get to higher orbitals (by absorbing energy), but right now I can’t think of any (unless you want to go into things like nuclear electron capture, in which a low-energy electron is absorbed by the nucleus and any higher-energy electrons can then drop down an orbital, but getting into nuclear chemistry here would just get messy :slight_smile: )

Is a photon the only thing that can move at the speed of light? And a photon will act like a wave or a particle under differenet observations, correct?