What does an e.m. wave consists of? On what factors does its velocity in vacuum depend?

Explanation

EM wave are those waves in which there are sinusoidal variation of electric and magnetic
field vectors, at right angles to each other as well as at right angles to the direction of its propagation.
These two field vectors vibrate in the same phase and with the same frequency.
The velocity of e.m. wave in free space is given by `c=1//sqrt(mu_0in_0) =3xx10^8 m//s`
where `mu_0` is the absolute permeability of space and `in_0` is the absolute permitivity of space.