  The gravitational redshift forms the central part of the majority of the
classical tests for the general theory of relativity. It could be successfully
checked even in laboratory experiments on the earth's surface. The standard
derivation of this effect is based on the distortion of the local structure of
spacetime induced by large masses. The resulting gravitational time-dilation
near these masses gives rise to a frequency change of any periodic process,
including electromagnetic oscillations as the wave propagates across the
gravitational field. This phenomenon can be tackled with classical
electrodynamics assuming a curved spacetime background and Maxwell's equations
in a generally covariant form. In the present paper, we show that in a
classical field-theoretical context the gravitational redshift can be
interpreted as the propagation of electromagnetic waves in a medium with
corresponding conductivity $\sigma=g/(\mu_0 c^3)$, where $g$ is the
gravitational acceleration and $\mu_0$ is the vacuum magnetic permeability.
Moreover, the energy density of the wave remains proportional to its frequency
in agreement with Planck's postulate.
