The Doppler Shift

• If a light source is moving with respect to an observer (or the other way around) strangely enough the Speed of the light being emitted doesn't change but the Frequency and Wavelength do.

  

• The waves get "bunched up (higher ν, shorter λ) in the forward direction and "stretched out" (lower ν, longer λ) in the backwards direction.

• Everybody know this effect in sound waves. (Think about the train whistle pitch as the train zooms past).

• The change in frequency is called the Doppler Shift

  λo - λv	=	v
  λo		c

  Where:

  • λ_o is the "at rest" wavelength;
  • λ_v is the apparent wavelength for the velocity v.
  • v is the relative velocity between the source and observer and
  • c is the speed of light.

Doppler Shift Example

• You are busy talking on your car phone and drive right through a red light. In court you claim that the RED light looked GREEN to you because you where approaching the light and the radiation was Doppler Shifted into the green part of the spectrum.

λo - λv	=	v
λo		c

(λo - λv)	x c = v
λo

(6000 Å - 5000 Å)	x c = v
6000 Å

Bad defense.