in 1905 Albert Einstein predicted the existence of photons as an explanation for long known physical observations
Albert Einstein
1879 - 1955
The energy of individual electrons increases with
the distance from their nucleus and it takes energy
to bring an electron from a lower to a higher band or
to knock it out of the outer (conductive) band. In a
conductive solid the electrons in the outer bands
jump from one nucleus to another but since they
effectively remain in the same band this requires a
relatively small amount of energy. There are
circumstances however that may force an electron
to move from a lower to a higher band. Such an
electron is said to be excited. Within a gas the
binding forces between individual atoms is relatively
small and electrons can quite easily be forced to
leave their initial band, for instance in a collision
between two atoms or between an atom and an
already freed electron. When an electron is excited
in such a collision and moves from a lower to a
higher band or even leaves its nucleus completely, it
absorbs energy in the process of the collision. The
electron that escapes from its nucleus, leaves an incomplete atom behind and this
atom then becomes positively charged. Such an atom is called an ion. In the
opposing relaxation process, an electron returns to an ion or descends from a higher
to a lower band while it expels the energy it absorbed before. The amount of energy
absorbed or expelled in a transition, differs per level and per element but is always
the same for a certain type of transition. An electron that expels energy does so by
emitting small packets (or quanta) of energy that are called photons. Photons hence
are carriers of energy and they neither posses charge nor mass. Their physical
behaviour is partly that of particles and partly that of waves or radiation. The
theoretical existence of photons as an explanation for long known physical
observations was predicted by Albert Einstein in 1905 and practically proved