How does the Doppler effect shift light's wavelength?
Sound waves are compressed when the source is approaching the listener resulting in the pitch rising and expanded when moving away lowering the pitch. The color of stars moving toward us shifts toward blue and moving away shifts toward red. However, since a photon carries the electromagnetic frequency, and the velocity of a photon does not change, how can the direction the photon moves in relation to the observer effect its wavelength?
This question is in the General Section. Responses must be helpful and on-topic.