The Doppler effect is the change in frequency or wavelength of a wave due to relative motion between source and observer, applied to radar speed measurement.
Extract
[...] Exercice 2 - Doppler Effect 1. The Doppler effect is the apparent change in frequency or wavelength of a wave when a source moves relative to an observer or vice-versa. If the source approaches the observer, the perceived frequency increases and the wavelength decreases. Conversely, when the source recedes, the perceived frequency decreases and the wavelength increases. 2. We have the following wave length schemes and : 3. The relation linking frequency, wavelength and speed is: where is the frequency, the wavelength and the speed. [...]
[...] On the principle scheme of the Doppler effect radar as follows: 9. The coefficient is due to the fact that the wave undergoes two times the Doppler effect: - once when it is emitted by the radar and reflects on the moving vehicle - a second time when it returns to the radar after reflection. 10. On a so . Application numérique : m/s or km/h. 11. With a tolerance of , the maximum allowed speed is km/h. The driver will therefore be penalized as well above this speed limit. [...]
[...] So let the wave . For a point located at from the source, after , son amplitude (i.e. its position) will be m. Let it be an expression wave . For a point located at from the source, after , son amplitude (i.e. its position) will be . Given that this wave produces a displacement corresponding to half the amplitude. We deduce that: so, . m/s. 4. On has two waves of amplitude propagating in the same direction with a phase difference . [...]
