# In an atom, when an electron loses energy, why is a photon released? If photons are massless, how are they created in this process and why?

This is my first question and I am just a 14 year old so excuse me for my mistakes. Please simplify your answer a little only. Using terms thinking I won't understand is a mistake.

• When an electron looses energy, this energy must go somewhere else. It is sent away. It turns out that it is sent away as radiation. Then we give such a "radiation energy chunk" a name: photon. – Steeven May 29 '17 at 6:52
• Just a comment since there are some excellent answers here, but the photon is only massless in its own "referance frame" - for all of us other observers, the photon has mass - it has a momentum. If you send a photon against a screen where it deflects, the screen will "feel" a force. It has to be so, because not only is energy conserved, but momentum is as well, and the electron needs to change both energy and "velocity" when it changes "orbit". The "velocity" change gives the photon its direction. – Stian Yttervik May 29 '17 at 13:31
• Maybe this is too much for the OP, but @StianYttervik no, wait. From a modern physics point of view, the photon is massless in any frame (and "its own frame" is ill defined), even though it has momentum. From a Newtonian point of view (no special relativity, no quantum mechanics -> no photon creation, no atoms as we know them), it makes much more sense, for the reasons you say and for gravitation, if the photon has some very small mass, in any frame. In both cases, the mass is always the same in any frame. – Effervescenza Naturale May 29 '17 at 14:37
• This one has answers you seek, and they are simple: physics.stackexchange.com/questions/143652/… they key is formula for energy. – luk32 May 29 '17 at 16:31
• And there I thought that photons had zero rest-mass, but did have mass as a consequence of their momentum. – Chris Becke May 30 '17 at 5:55

We believe that there exist something called an electromagnetic field, this is an invisible interaction, just like gravity that pulls you down, the electromagnetic field is a force that acts on magnetic matter and electric charged matter.

The photon is an excitation of this field. Think about it this way if the electric field was a big sheet of cloth, the photon is the waves that happen on the surface. The electron can create photons because the electron is electrically charged so it can create an electric field around it, and if the electron moves quickly or oscillates, it will radiate these waves (photon), just like if you take one part of the cloth sheet and move it quickly or move it Up and down, you'll see a wave going out of the place where you move it.

I tried to be very brief but please ask me for more clarification if needed.

• What you said actually makes sense to me. But sir, I have read about a theory of higgs field where they say that everything interacts with it and gives mass to objects, it also says that for some reason photons do not interact with it and thus they don't have mass. Who is correct you or the higgs field theory? – Rajdeep Singh May 29 '17 at 15:12
• You're right the photon does not have a mass because it's not coupled to the Higgs field, but this does not cancel what I told you about how a photon get created. Both description are correct. – Ismasou May 29 '17 at 15:22
• " Invisible" in the first sentence is a bit funny. Gravity may be invisible, and yes, some electromagnetic fields are invisible, but all visible fields are electomagnetic. That pretty much follows from the definition of "vision" ! – MSalters May 29 '17 at 15:35
• you never see the Faraday line, you only see excitation of the field, well yeah you can visualize these lines using a magnet and some powder steal.. But the lines are an abstract mathematical object, that things interact with. – Ismasou Jun 3 '17 at 19:40

A simple picture:

From classical electromagnetism we know that an accelerated charge radiates electromagnetic waves.

One of the reasons we arrived at the quantum mechanical model of the atom is because, classically, an electron rotating around a positive charge, because of the radial acceleration would radiate a continuous spectrum until it fell on the positive charge and neutralized it. This evidently did not happen because atoms, as the simplest one, the hydrogen atom, exist and are stable.

When an electron fell onto a proton to form hydrogen, observations showed a specific energy spectrum was radiated

Balmer series

which can be be fitted with mathematical series , at discrete frequencies.

This led to the Bohr model of the atom, where to explain the series, it was postulated that there existed orbits of the electron that were stable unless they were excited with an incoming radiation, and the lines seen were transitions between orbits.

At the same time, Planck postulated the photons, discrete particles which built up light with frequency h*nu, to explain black body radiation.

The discovery that the Shrodinger equation could give solutions for the hydrogen atom electric potentials and they fitted the hydrogen spectrum, tied the whole thing up.

Photons are the quantum of the electromagnetic spectrum, and they are absorbed or radiated in interactions with atoms because the atom is mainly held together by an electric potential, which gives quantized energy levels , and not the continuum of classical theory which had no photons.

There is a big problem with quantum mechanics: it is highly abstract and sometimes counterintuitive when you are about to go deeper. Don't worry, anybody interested in quantum mechanics had to face this. The fact, that you are concerned by this is the good sign that you will pass successfully.

The massless photon is massless in terms of stationary mass. The whole energy is in form of kinetic energy in terms of Enistein's special relativity. When the photon is stopped, its whole energy is transformed in any other form and it's energy comes to zero, co it ceases to exist at all. The photon, actually, is just a theoretical construct to trick our minds to be able to digest all the properties of light.

The photon, at its origin, is just a theoretical construct how to describe the Planck's quantum theory where the energy is not radiated via continuous energy stream but in small chunks. Those energy chunks were called the photons.

You know that ideal pendulum stores mechanical energy in forms of kinetic and ptential energy. You know that connected pendulums can transfer the energy from one point to another. Try it with your friend. Hold a rope on both ends, strain it a little and then swing the hand holding a rope. You will see a hump running towards your friend and finaly swingign his hand. If we consider the rope the electromagnetic field the hump is the photon. The problem with the cloth example is that the photon-hump is localized both in distance from you and direction from you. It looks more like a ball sent from you somehere when the observer is below (and can see only the cloth shape). And the hum doesnt represent the actual displacement but the amplitude of the oscilations in te position of the hump.

The hump wasn't there when you swung your hand; it appeared (like photons do when excited atom comes to the stable level) and it disappeared when it hit your friend (like photons do when they hit atom exciting it).

• Thank you sir, your answer was helpful. But, how do you explain the photons coming from sun, I know this much that at the core sun has nuclear fusion in which hydrogen is fused into helium but how does the photons or light come to us? How does the photon reach to us through the core ? – Rajdeep Singh May 29 '17 at 15:01
• In the case of the Sun it is very complicated; there is really hot and really dense plasma there; it is believed that photons we can see are from relatively thin surface layer. Near the core the plasma is really dense and there is high probablity of interaction of photons and other particles. – Crowley May 29 '17 at 15:23
• In the ideal vacuum one can suppose the electric and magnetic fields to be constant, say zero. When the photon passes by it means that in higly localized part the $E$ and $H$ vectors oscillates and the point where the amplitude of such oscilations is at maximum moves in one direction. If such chunk hits electron, this field distotrion interacts with the electron changing its state. Note, that electron itself is not a green ball at all, it is a chunk of probability wave where the carge may be localised. When the electron changes its state it "shakes" the field causing a wave chunk to appear. – Crowley May 29 '17 at 15:33

One shouldn't think of a field as a surface, a field is a mathematical computation of a phenomenon. It is an abstraction. Really what the field is, is a force at each point. When the source of a field jiggles, then the realignment of the force values at each point happens in a motion that can be approximated to a mathematical wave. But this realignment carries an energy in and of itself, which is the energy associated to a wave. One can see that this is all mathematical mumbo-jumbo for a physical phenomenon that is hard to imagine.

A photon, again, is only the "unit", so to speak, of this realignment. Clearly it cannot have mass because it's not an object, it is an abstraction.

• So, the photon is not a particle but actually a wave that is released and this WAVE is called a photon. My only question now is, a photon has electromagnetic energy and not kinetic energy ? – Rajdeep Singh May 29 '17 at 14:45
• The readujstment changes the force values at each point, altering the potential (which is nothing but potential energy) at each point. Hence the energy put into jiggling the source of the field is communicated to the field in this manner. What you call it is just a matter of label, I suppose it would classically be called wave energy. Kinetic energy is associated to the movement of objects, doesn't apply here. Electromagnetic energy is total potential energy in a field, so I suppose you could consider it such, because it changes the EM energy of the configuration. – Steve May 29 '17 at 14:59
• A field is basically a system but at atomic level. When you say a field, I imagine it to be a system. I am in 9th standard and we use the word system. Is there any difference between the both ? – Rajdeep Singh May 29 '17 at 15:22
• No, a field is a vectorial function en.wikipedia.org/wiki/Vector_field. Every point in space has an associated vector. It is a mathematical formalism. For example, water travelling through a pipe, the velocity vector field would be that at every point, the tiny piece of water around the point has velocity V pointing in a certain direction, and it varies by point. The electric field is a vector field such that the force generated by a proton or electron in space will depend on the point. Since force is a vector, it can be represented as a vector field. – Steve May 29 '17 at 15:26

If you release the potential energy stored in a piece of water that is mechanically lifted above the water surface level, the energy will radiate in the form of waves. These waves carry away energy, but are massless.

In an atom, the potential energy from moving the electron in the Coulombic potential (the attraction of an electron and a proton) is also released as a wave, but the force that causes the potential energy is electromagnetic and the resulting wave is electromagnetic.

Now, because of quantum mechanics there are discrete energy levels and light quanta - photons, but it is still an electromagnetic wave.

• Correct me if I am wrong, When an electromagnetic wave hits a electron, it reaches a higher energy level. This elestron releases this energy because (I think) all particles want to remain at a stable state where they have the lowest energy. – Rajdeep Singh May 29 '17 at 14:46
• No, I just wanted clarification. – Rajdeep Singh May 30 '17 at 7:07
• Sorry, I didn't realize you were the original asker. Yes, sure. It is quite similar, but there are also differences. With the water wave you have a piece of water above the surface. It has higher energy, so it falls down and waves emerge. In an atom the electron falls down closer to the nucleus and an electromagnetic wave emerge. The similarity does not go much farther, the water surface in gravity is different in many aspects, but some principle remains similar. – Vladimir F May 30 '17 at 7:50

You are right, photons are massless and is only a form of energy.

Since electron on getting energy goes to higher state but becomes unstable simultaneously. So it rejects the extra gained energy to achieve its stability.

Now, this energy has to go somewhere and experimental data shows that on achieving stable states light bands of equal frequency is produced. Equal energy suggest that no conversion of energy or loss has taken place.

Since bands were discrete then some discrete particle is being emitted. And photons are the dicrete particles present in light constituting only energy. Thus photons are produced.

NOTE: If you are unaware of or doubt any term, read about them which will help you resolve your doubt.

That's just kind of how it goes. Conservation of energy plays a role as well. If the electron loses energy, it has to go somewhere, thus, the photon with energy.