Red-shift, blue-shift and Doppler effect. The Doppler effect causes the light waves emitted by a source, such as a galaxy (centre), to be red- or blue-shifted when the source is moving relative to the observer. This is seen by looking at the absorption or emission lines in the spectra of such objects. In the spectra (bottom and top) four dashed lines show their positions if the source is not moving. In the spectrum at top (source approaching the observer) the four lines move towards the blue part of the spectrum and are shown as solid lines; in the spectrum at bottom (source receding from the observer) the four lines shift towards the red part of the spectrum.