Full derrivation of Doppler effect shows the observed wavelength changes for moving observers and wave speed does not
COPILOT: Setup: 1D wave, fixed medium, moving observer Take the simplest case: Medium: at rest, defines the preferred frame. Wave: propagates in +x with speed c in the medium. Source: at rest in the medium, emits crests with period T , frequency f = 1 / T , wavelength λ = c T . Observer: moves along +x with speed v (away from the source), measured in the medium frame. We want λ ′ : the distance between successive crests at the same time in the observer’s frame . Step 1: Crest emission and motion in the medium frame In the medium frame: Crest n is emitted at time t n = n T Its position at time t is x n ( t ) = c ( t − t n ) = c ( t − n T ) The observer’s position is: x O ( t ) = v t Crest n reaches the observer when: x n ( t n ′ ) = x O ( t n ′ ) So: c ( t n ′ − n T ) = v t n ′ Solve for t n ′ : t n ′ ( c − v ) = c n T ⇒ t n ′ = c c − v n T This is the arrival time of crest n at the moving observer. Step 2: Observed period and frequency (Doppler) The observed period is ...