  By exploring the relationship between the propagation of electromagnetic
waves in a gravitational field and the light propagation in a refractive
medium, it is shown that, in the presence of a positive cosmological constant,
the velocity of light will be smaller than its special relativity value. Then,
restricting again to the domain of validity of geometrical optics, the same
result is obtained in the context of wave optics. It is argued that this
phenomenon and the anisotropy in the velocity of light in a gravitational field
are produced by the same mechanism.
