PX154 - G6 - the doppler effect

the apparent change in frequency as measure by an observer when the source and/or the observer are moving relative to each other
- eg:
  - change in tone as a vehicle passes
  - redshift of galaxies
  - radar guns in speed traps
we have to treat sound and EM wave differently because:
- for sound, the speed is fixed relative to the atmosphere
  - we have to consider the motions of both the source and receiver relative to this
- EM waves: just need the relative speed of the receiver relative to the source

f^{'} = f (\frac{v + u_{0}}{v + u_{s}})

where, $f^{'} =$ observed frequency, $f =$ source frequency, $u_{0} =$ velocity of observer, $u_{s} =$ velocity of source

always be careful about signs:
- $f^{'} > f$ : moving closer
- $f^{'} < f$ : moving away
compare with EM waves:

f^{'} = f \sqrt{\frac{c - v}{c + v}}

where, $c =$ speed of light, $v =$ the relative speed of source and observer