Properties of a Wave
The distance between crests or troughs is the wavelength l. If the wave
travels at speed c, the crests will pass with a frequency f. The relation between these is
 l x f = c.
People wondered what “medium” the light waves are traveling in. Today we just say it is the vacuum (which isn’t totally empty in modern physics). We also now know that the speed in this medium is the same no matter how fast the emitter is traveling (very odd: a result of Einstein’s relativity).