According to wikipedia, electromagnetic waves are "synchronized oscillations of electric and magnetic fields that propagate at the speed of light". I understand what it means in theory. But in practice we don't have a uniform EM field in nature, do we? EM field near a power station and in outter space might be quite different, does it influence the way EM waves (I'm thinking of light) propagates?
So in short: how is light influenced by EM fields on earth and what are the properties of EM field in absolute emptyness if that exists (in particular what is the intensity?).