Doppler Effect: this UHD 4K animation shows how the apparent 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. This effect can be noticed 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 an orange cone as the wave source. As the cone moves from left to right it emits wave-fronts that continue to radiate out from their position of origin. The cone moves on and establishes a new point of origin for the next wave-front and so on. In this way the waves bunch together at the front of the cone and spread out behind. The situation illustrated is 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 undergoes a red shift.