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
- 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 is
the "at rest" wavelength;
- λv 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
Q. How fast would the judge calculate you were
- 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
|(6000 Å - 5000 Å)|
| x c = v
= 110,000,000 miles/hour