Doppler effect

• When the source of waves, such as sound waves, moves towards or away from an observer, there is a change in its observed frequency. If you hear a police car or an ambulance with its siren sounding, you will notice that when the source of sound is coming towards you it has a higher frequency (pitch) than when it is going away from you. This effect is called the Doppler effect, and the change in frequency is called the Doppler shift.
• The diagram above shows a motor cycle travelling from left to right. The lines show the wavefronts as the sound of the motor cycle travels through the air. For the boy the wavefronts are pushed together more because the motor cycle is travelling towards him and the sound he hears has a shorter wavelength, or a higher frequency, than the sound heard by the girl.
• All waves demonstrate the Doppler effect when the source is moving relative (towards or away from) the observer, but the effect is less obvious when the speed of the source is very small compared with the speed of the wave.