Let us list some of the factors, which could possibly influence the speed of wave propagation:
(i) nature of the source.
(ii) direction of propagation.
(iii) motion of the source and/or observer.
(v) intensity of the wave.
On which of these factors, if any, does
(a) The speed of light in vacuum
(b) The speed of light in a medium (say, glass or water), depend?