A photon is a quanta of light. Our picture of light, to this point, has been that of a wave. Wave-like characteristics are responsible for diffraction and refraction. However, light is absorbed and emitted one photon at a time. The energy in a photon is related to the frequency of the light wave through Planck's constant. Photons are
massless particles. The idea of massless particles may
seem a bit strange. To understand them better, we
consider the expressions for energy and momentum of
particles of mass Massless
particles can be thought of as the limit of massive
particles with g may approach
infinity as the velocity approaches .
Thus for massless particles, the above expressions are
meaningless, but the ratio of the two expressions is
valid as the factors cg
and
cancel.mWe have made use of
the fact that the velocity for massless particles. By
adding the usual relation of wavelength and frequency, c
= cl, one
gets three useful expressions relating energy, momentum,
frequency and wavelength.fThus light of
shorter wavelengths, interacts as photons with higher
energy than does light with longer wavelengths. We will
later discuss the fact that the latter two expressions
remain true for massive particles, although the energy in
the central expression includes the rest energy, (1/2) |