The Doppler effect – named after Austrian physicist Christian Doppler – is the perceived difference between the frequency at which sound, or light, waves emanate from a source and at which they are received. A good everyday example is an ambulance approaching a pedestrian. The vehicle’s siren emits sound waves at a certain frequency, which if both the ambulance and person were stationary would remain at a constant pitch. But, as the vehicle speeds towards the person, the pitch gets ever higher until it passes, lowering again as it recedes. The reason for this shift is that, due to the motion of the ambulance, the sound waves bunch up, taking less and less time to reach the pedestrian’s ears . This bunching up effect increases the number of waves reaching the receiver at once, leading to an apparent fluctuation in pitch.