The Doppler Shift – how it works
When a source is moving, an observer gets
the waves either stretched out or crunched together, depending on their
relative motion with the source. In the case of light, longer wavelengths
look redder and shorter wavelengths look bluer. This is given by the Doppler
formula:
v is negative for an approaching source:
if the distance is shrinking, the wavelength is too
To get an appreciable change, you have
to be moving
with an appreciable fraction of the speed of the wave