Divisibility of Photons
Since the energy of a photon is quantized, does it mean
that its energy is not divisible?
That is exactly right. Quantization means that it is the smallest discrete
entity--that there exists no smaller elementary particle.
An isolated photon, say coming from a distant galaxy can travel inter-galactic
distances without having its energy change. The energy given by: E = h x (nu)
where E is the energy, (nu) is the frequency and 'h' is Plank's constant.
However, a photon that interacts with matter by various mechanisms such as,
scattering, absorption, reflection, and many others. Then the photon loses its
identity. For example, you cannot put a tag or label on a photon that is
absorbed by an atom or molecule. Other photons of the same or lesser energy
(lower frequency). However, there is no way to know that those emitted photons
are the same as the incident photon. This is a general rule.
For example a neutron can decay into a proton and an electron (and also an
anti-neutrino). But the neutron is not simply the sum of the particles. As long
as the electron, proton, and anti-neutrino reside in the neutron they do not
have separate identities. They are in fact a combination of quarks held together
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Update: June 2012