The Doppler Effect

- If a wave source is moving relative to an observer, there'll be a change in the observed frequency and wavelength due to the Doppler effect. This occurs due to the bunching or spacing of the wavefronts.

- Higher frequencies produce shorter wavelengths, assuming the wavelength does not change.

- As an ambulance hurries past a pedestrian, the pitch of the siren changes.

- As the ambulance travels toward the observer, the wavefronts become closer together.  This shortens the wavelength and increases the frequency, so the sound appears to have a higher pitch.

- As the ambulance travels away from the observer, the wavefronts become further apart. This lengthens the wavelength and decreases the frequency, so the sound appears to have a lower pitch.

Examples

1) A ferry blares its horn as it sails out of a port at 10 m/s. The captain driving the ship hears a note with a wavelength of 3.3m. If the speed of sound is 330 m/s, calculate the λ of the note, someone waving at the boat standing at the port.

2) A bumblebee's wings make a humming sound with a frequency of 55 Hz. As it flies past, you hear the sound at 57 Hz. Calculate the change in wavelength. 

3) A police car siren emits a tone with a wavelength of 2m. It's travelling at 40 m/s and the speed of sound in air: 330 m/s; calculate Δλ.