  The question we ask is: does the speed of light {\it{in vacuo}} depend on its
frequency? While the answer is NO in the frame of classical physics, we point
out that the opposite could be true if one takes into account the polarization
of Dirac sea. We estimate the dependence of the index of refraction of vacuum +
Dirac sea versus the wavelength of an incoming beam, and suggest a way to test
this effect.
