  The concept of photon is not necessary only applied to the relativistic
Doppler theory. It may also work well for classical theory. As conservation of
momentum and energy are physical laws, if applying these laws gives the exact
relativistic Doppler effect, it should also give the exact classical Doppler
effect. So far the classical Doppler effect is only obtained by using some
approximation, as derived by Fermi in 1932. We show that the exact classical
Doppler effect can be derived from the photon emission process in the exact
treatment and reveal that these results are the same as those derived from the
wave theory of light.
