(c) for all intensitiesFor any given medium, the speed (c) of an electromagnetic wave is given byc=vλ,  wherev= frequency of the electromagnetic waveλ= wavelength of the electromagnetic waveAs the frequency and wavelength are changed, the speed of the electromagnetic wave changes. So, the speed of an electromagnetic wave is not same for all wavelengths and all frequencies in any medium. The velocity of an electromagnetic wave changes with change in medium. Also, the speed of an electromagnetic wave is same for all the intensities in any medium.