# What happens when a photon "dies"?

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.

A photon is just a wave of change in the background electromagnetic field. Or, to be more precise, it is a packet of electromagnetic energy that is irreducible without altering its frequency of oscillation.

Knowing this, a photon is "born" when one form of energy is converted into electromagnetic energy (e.g. from an electron's potential energy when transitioning from the excited state to a lower state or from thermal energy in blackbody emission or from mass energy in particle annihilation). A photon, conversely, "dies" when its electromagnetic energy is transformed into another form of energy. Some examples of this could be through the excitation of an electron in an atom, it could be absorbed and turned into thermal energy, it could be used in particle pair production, or it could be turned into mass when falling into a black hole.

That's it. Knowing a photon is just a packet of energy means that the rules of energy conservation apply. Energy cannot be created or destroyed*, merely transformed into another form. So a photon can be transformed into other energy and, thus, "die" by any means we might use to convert energy from one form to another.

$$^{\text{* this is locally true}}$$

Start with the electromagnetic spectrum which has freqencies from very small to very high. The visible spectrum, color you call it, is a small part of the spectrum. Electromagnetic waves are described by the classical Maxwell equations.

Then we found that everything at the microscopic level follows quantum mechanics, and the particle photon builds up in superposition with zillion others of the same energy to make up the classical electromagnetic wave , art of which is the color spectrum our eyes see.

Then biology comes in, and color perception. What our eyes call red, is not the red in the spectrum ( the rainbow for example has the pure frequencies). It is called color perception. Our eyes call black the absence of the perceivable colors, but the object called black still radiates photons according to its temperature.

Photons are created in two ways, and always quantum mechancis holds.

a) when a charged particle accelerates or decelerates, interacting with a field, magnetic or electric, a photon comes out , with a probability given by quantum mechanical calcualtions

b) what you describe, individual atoms in excited energy levels given by the quantum mechanical solutions, can deexcite and give photons. When photons of that energy meet an atom, they can scatter and excite it to a higher level, thus the photon is absorbed and "dies".

c)more generaly in matter which is composed out of quantum mechanical entities, atoms, molecules, lattices of molecules, the solution of the equations defines fixed energy levels for the electrons/nuclei/atoms/ molecules/ lattice positions. Because all matter has a specicif temperature, and temperature is connected with kinetic energy, the motion of these quantum charged entities generates a spectrum of photons from excitations and deexcitations, called black body radiation. The absorption is the "death" of that photon.

The color we see as black, means that it is absorbing visible photons, picking up energy. Thus a black surface in the sun is hotter than a white one, which reflects visible light.

So it is not just bouncing but also interacting in various ways with the matter on its way that a photon "dies" .