Apologies if my question is unclear, any help to clarify it along the way is most welcome.
I'm confused about what we mean when we say electromagnetic 'waves' (say visible light). In the usual mental picture we have of a simple sine wave, what does the y-axis correspond to? In mechanical waves like water or sound, we can plot the vertical displacement of each particle along the x-axis as a value on the y-axis. Alternatively we can fix a specific particle in the water/air and take the x-axis to be time and the y-axis to be its physical displacement.
But for light traveling in a vacuum, there would be no such displacement of particles. So what does the y-axis correspond to? What do we mean by light being/behaving like a wave?