How to explain photon is quanta of light? How do you explain photon as quanta of light?
I thought of photons as quanta of light which are the smallest unit of light.
But then I learned a photon can be split into two or even three photons (red-shifted, energy is conserved), and also photon can lose energy and still be a photon (Raman effect, inelastic scattering).
Now, I am not sure what it means when it is said photons are quanta of light (smallest unit of light).
Could somebody please enlighten me?
 A: Photons are quanta of light at any given frequency and corresponding wave length. If there is a laser beam at wave length $\lambda$, it consists of photons which each carry an energy of
$$
E_\gamma = h \nu = h \frac c \lambda~,
$$
where $h$ is Planck's constant, $\nu$ is the frequency and $c$ is the speed of light. It is impossible to emit any light of wave length $\lambda$ if there is less energy available than $E_\gamma$. Likewise, if some physical system absorbs some light of wave length $\lambda$, it will gain an energy of $nE_\gamma$, where $n$ is a natural number.
A: nu’s answer is correct. Conceptually, the answer is that the photon is the minimum amount of light at a certain frequency. If it loses energy, it goes to a lower frequency. The energy is proportional to the frequency. If a photon splits into photons (without energy added or subtracted), the frequencies of the new photons will add up to the frequency of the original photon to conserve energy.
A: In contrast to the fermions from the standard model of particle physics, whose energy contents are constant values (their rest masses), the photon is a bundle of particles with very different energy contents.

How do photons come to life?
The only possible way to generate electromagnetic radiation is through excited emission from atoms, molecules and compounds. Excited by incident EM radiation, electrons in particular fall back into their equilibrium and emit photons.
In dependence from their bounding states in atoms the emission is partially in relation to the incoming photons (Raman effect, inelastic scattering, …). Some energy is dissipated by the momentum of the photon and lead to vibrational energy in matter.
BTW this is thermodynamics, which states that any process is accompanied by energy losses to heat energy (infrared radiation).
How do they end their existence?
Only through the absorption of the particles described above.
What is in between?
Photons are indivisible particles between their emission and their absorption.
If electrons get excited synchronously and in the same direction, the emitted photons are polarised and they act on matter differently as EM radiation from thermic sources.
The best example are radio waves. They induce in the receiving metall a lot of electrons at once and this current allows us to transmit information from the emitting antenna to the receiving. Of course in this case the re-emission is different as from unmodulated EM radiation.
Furthermore artful designed layers of material lead to very different re-emissions. Be this a painting in black or white or any color or be this the stealth technology for war maschines.
