The 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 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.