Electromagnetic wave
In physics, an electromagnetic wave is a change, periodic in space and time, of an electric field E(r,t) and a magnetic field B(r,t). In the figure we see a snapshot (i.e., a picture at a certain point in time) of the magnetic and electric fields in adjacent points of space. In each point, E is perpendicular to B. The wave propagates to the right, along an axis which we conveniently refer to as x-axis. Assume the snapshot is taken at time t, then at a point x we see an arrow of certain length representing E(x,t). At a point in time Δ t later the same value of E (same arrow) is seen at x + c Δ t. The arrow seems to have propagated to the right with a speed c. For all kinds of electromagnetic waves (see below) the propagation speed c is the speed of light.
To be continued