I have a question. I've been told that expansion "stretches" the light out and causes the redshift. However, the frequency of the light is the number of oscillations in its fields per second correct? How does "stretching" the wave affect the frequency? I know that v=fλ is the equation relating frequency to wavelength, and that increasing the wavelength should decrease the frequency, but is that really all there is too it? Or is it due to the recession velocities similar to normal doppler shift? (Which, for an EM wave still doesn't make sense to me) Perhaps a better question is how does red/blue shift (for whatever reason) change the frequency of the light? Is it simply in the math and the above equation? I can understand the doppler effect of sound, as each wavefront takes longer to arrive if something is heading away from you, and quicker if it's heading towards you, but I have a hard time understanding this effect on light.