Perhaps this is a misconception, but why are electrons alike and photons not? Given two photons, they may differ by having different frequencies (energies). Given two electrons, there are just two indistinguishable electrons?
[Physics] Why are electrons alike but photons not
electronsidentical-particlesparticle-physicsphotons
Related Solutions
One good piece of evidence that all particles of a given type are identical is the exchange interaction. The exchange symmetry (that one can exchange any two electrons and leave the Hamiltonian unchanged) results in the Pauli exclusion principle for fermions. It also is responsible for all sorts of particle statistics effects (particles following the Fermi-Dirac or Bose-Einstein distributions) depending on whether the particles are fermions or bosons.
If the particles were even slightly non-identical, it would have large, observable effects on things like the allowed energies of the Helium atom.
This is a question coming from somebody who has had at most some high school physics. It would help if in your profile you added your age and/or field.
From the level of the question I will assume the above.
Explanations I have read of why photons are emitted from atoms mention electrons being 'excited' to another energy level, and then returning to their base level, releasing a photon.
This comes about because the microcosm is quantized, i.e. obeys the rules of quantum mechanics, and at a first level of explanation,the nucleus of the atom is a potential well into which the electron, when speaking of hydrogen, can have only quantized specific orbits. The orbits are defined as energy levels. If, for some reason, the electron is at a higher energy level and there exists a level below, it falls to that level releasing a photon which has energy h*nu equal to the difference in energies of the two orbits. Usually the excitation comes about from a kick to the lowest energy electron of an appropriate energy to send it to a higher energy orbit, or to ionize the atom. For atoms with a higher atomic number, the energy levels are filled up sequentially with electrons which again can behave as above.
I have also seen the occasional mention of 'fields' and I vaguely expect from what I've read that interaction of the electrons with these fields has some effect.
In the above, first level picture, the field is the electromagnetic field, and mainly the effective field coming from the charges of the nucleus and the orbiting electrons.
I've also seen, but not used, Feynmann diagrams that involve additional particles.
Feynman diagrams come into the next level of complexity of the theory of elementary particles. The description above pertains to first quantization. Feynman diagrams belong to second quantization and are a powerful and necessary tool for calculating processes in elementary particle physics, like electron electron scattering etc. This needs a course in Quantum Field Theory.
Best Answer
It's a good question, and one that puzzled me for a while as well. However the answer is very simple.
For a massive particle like an electron the total energy is given by:
$$ E^2 = p^2c^2 + m^2 c^4 $$
where $p$ is the momentum and $m$ is the rest mass of the electron. Electrons can obviously have any momentum you want, so the total energy can be any value greater than $mc^2$. The de Broglie wavelength of the electron is $\lambda = h/p$, so the electron can have any wavelength you want.
If we now consider a photon, the key difference is that the rest mass is zero, so the equation for the energy becomes:
$$ E^2 = p^2c^2 $$
Just like the electron, the photon can have any momentum you want, so the total energy can be any value greater than zero. The wavelength of the photon is again $\lambda = h/p$.
So there isn't any difference between the electron and photon except that the non-zero rest mass of the electron means the energy can't be zero. Both electrons and photons can have different energies and wavelengths.