How the wavelength of wave is measured? My idea of electromagnetic wave is there is an invisible line traveling through space moving up and down like a sine wave we see on CRO. Is this idea correct? Or is it the the variation in electric and magnetic field of the wave that we relate this wavelength with and there is no such invisible line? Please explain. I have searched over net for electromagnetic wave, all pics shows two sine waves(representing magnetic field and electric field) perpendicular to each other. Is this how the wave would look like if we can see it?