As far as I understand a photon is produced, or "born", whenever an electron moves from a high energy state back to its normal energy state.
The photon then travels at the speed of light across space in a straight line until it hits another atom, or rather, interacts with the electron shell(s) of that atom. The energy signature of the photon can change at this point.
The photon can then bounce off that atom, and will continue to travel across space at the speed of light until it hits another atom. And so on.
(Please correct me if any of my understanding here is off.)
However what I want to know is what happens when light stops, and how this relates to the photon. I want to know what happens when a photon "dies" – not in a literal sense, just in the sense of when it has finished its journey of bouncing from atom to atom.
If you stand in a huge and pitch-black cavern, and shine a torch, the light will only carry so far. Am I right in assuming that the photons produced by the torch eventually stop bouncing from atom to atom, or does the journey of the photon continue and its just undetectable to human eyes?
Similarly, the colour black "absorbs light" – does this mean the colour black is "eating" photons? Does the energy get transferred to the electrons of the black material? What happens to this energy?
And finally, does the same "photon death" happen when a photon hits the retina in a persons eye?
In short, what happens when a photon dies?
Best Answer
This question is about the nature of the electromagnetic field. The electromagnetic field is a physical system that is most fully described by quantum field theory, and the results match those of classical field theory in certain limiting cases. The 'photon' is a physical picture which gives us a useful way to imagine certain aspects of this field. It is primarily a way to track energy movements.
The main thing you need to know is that energy is conserved, but photons are not. When energy moves from some other form to an electromagnetic form, then photons are created. When energy moves from an electromagnetic form to other forms, then photons are destroyed.
Another way of saying the same thing is to note that when an electron moves from a higher to a lower energy level in an atom, it does so through the way its charge pushes on the surrounding electromagnetic field, causing it to vibrate at a higher amplitude (the electric and magnetic parts both start to vibrate). This vibration, when it happens at a fixed frequency, can be conveniently modelled by saying it has a fixed amount of energy, equal to $h f$ where $h$ is Planck's constant and $f$ is the frequency. If this $h f$ is equal to the energy change $\Delta E$ in the atom, then we say one photon has been created. You can also find cases where two photons are produced, one at frequency $f_1$ and the other at $f_2$, and then $h f_1 + h f_2 = \Delta E$. This kind of process is much rarer but it illustrates that energy is conserved, but a given amount of energy can be expressed physically in more than one way.
Eventually a photon may arrive at some other atom and be absorbed. What happens then is that the oscillating electromagnetic field pushes on the electrons inside the atom, until one of them gains some more energy. The field vibration then falls away as the energy is transferred. We summarise the process by saying that the photon has been absorbed. Or, if you like, the photon 'dies'. This is just another way to say that the field has stopped vibrating.