The Doppler shift effect consists in the change of wave frequency registered by the receiver. The change is the difference between the wavelength in the situation when the relative motion of the source proceeds from or to the waves receiver and the situation when the source doesn't move. For light, as the source moves closer to the observer, the spectrum is shifted toward violet; as the source distances from the observer; the spectrum is shifted toward red. The shift of the spectral lines makes it possible to determine the velocity of the waves' source movement with the respect to the receiver (observer).
