Doppler effect and spectral line shifts. Illustration showing the bunching up of waves in front of a moving source. The source is moving from left to right. The waves behind the source are more spread out. In light sources this change in wavelength creates a blue-shift for an approaching source and a red-shift for a receding source. With sound sources it creates the change in pitch as the source moves past the observer. The absorption and emission lines in the spectrum also shift, as shown here.