Animation of the increase in frequency observed in waves emitted from an approaching source (or conversely a decrease in frequency in waves emitted from a receding source). Happens in sound waves (e.g. a siren of an approaching emergency vehicle or the change in pitch of a passing car engine) and in light waves (the red Doppler shift of a receding star or the blue Doppler shift of an approaching star).
This animation uses a ball as the wave source. As the ball moves from left to right it emits wave-fronts that continue to radiate out from their position of origin. The ball moves on and creates a new point of origin for the next wavefront and so on. In this way, the waves bunch together at the front of the ball and spread out behind. Similar to a jet plane approaching the speed of sound and radiating sound waves. In the case of light waves then the higher frequency at the front yields a bluer light (blue shift), while behind the light source (e.g. a star) the light is stretched out, lengthening the frequency, creating a redshift.