I had a thought experiment related to wave speed and frequency. Lets say we had a wave emitter that emitted transverse plane waves at a regular frequency. Of course we could imagine this as a radio tower, but I wanted it the thought experiment to be more general. Say the emitter is sending out the waves at a frequency of 100 cycles per second (cps). We'll set the wavelength to one meter and we'll set the medium of propagation to one whereby the velocity of the wave in this scenario is 100 m/s. So what we have is velocity=wavelength x frequency which equals 100=(1)(100) Now we have an observer Bob watch these waves come in at 100 cps and he gets comfortable seeing this steady, predictable display. After a while, though, we switch the medium between the emitter and Bob so that the speed of the wave is cut in half, down to 50 m/s. Subsequently, Bob now sees the waves coming in at 50 cps. My question is, is there any way Bob can tell that the wave has slowed down rather than the emitter instead just slowed down it's rate of emission? In other words, can Bob tell that speed of the wave has halved rather than the wavelength of the wave has doubled? Sure, he could ask the emitter tower to stop transmitting, wait a while, and then send a pulse and measure the speed of the pulse, but this thought experiment assumes that this is not possible, all there is a continuously oscillating wave moving through Bob's vicinity. Part B of the question is the extrapolation to light. I understand that light travels at the constant speed of c in all instances, and that this speed has been measured directly without regard to doppler shift. My question, though, is that, without those direct measurements and Maxwell's equations, etc., if we just had to rely on directly measuring a continuously emitting light source, is there any property inherent in that signal that would allow us to distinguish the velocity from the wavelength/frequency?