• Imagine an atom with excess energy. Without external action,
the atom would, after a period of time, spontaneously emit light,
whose properties (e.g. its direction) are random — only the
energy is fixed.
• This situation changes when the
atom is shone with a light wave of
the corresponding energy. There
is then a higher probability that
the atom will also simply copy the
remaining properties of the incident
wave: It then emits light that is
in step with the original and
propagates in the same direction....