Figure 3-5The Doppler Shift Wavelength is affected by motion between the light source and the observer. A source of light is moving toward the left. The four circles (numbered 1 through 4) indicate the location of the peaks of light waves that were emitted by the moving source when it was at points S₁ through S₄, respectively. Note that the waves are compressed in front of the source but stretched out behind it. Consequently, wavelengths appear shortened (blueshifted) if the source is moving toward the observer and lengthened (redshifted) if the source is moving away from the observer. Motion perpendicular to the observer’s line of sight does not affect wavelength.