Wave is just a graph of the intensity of energy at the given point in space right? At a particular point in space, we detect that energy is going up ad then down with each second- when we plot this on a graph against time we get a wave. And 'on the graph only' the wavelength concept is explained right? But with what instrument was this up and down of energy level detected? How did we know that there is an electric field and a magnetic field? Like sound waves travel by creating compressions and rarefactions of the air/gas molecules. How do other forms of energy like electromagnetic radiations actually travel?

What exactly are EM waves? Wave is just a graph of the intensity of energy at the given point in space right? At a particular point in space, we detect that energy is going up ad then down with each second- when we plot this on a graph against time we get a wave. And 'on the graph only' the wavelength concept is explained right? But with what instrument was this up and down of energy level detected? How did we know that there is an electric field and a magnetic field? Like sound waves travel by creating compressions and rarefactions of the air/gas molecules. How do other forms of energy like electromagnetic radiations actually travel?