What is the Doppler effect in sound?

The Doppler effect in sound is a change in frequency or wavelength of a wave in relation to an observer moving relative to the wave source.

The Doppler effect, named after the Austrian physicist Christian Doppler, is a phenomenon that occurs when the source of a wave (like sound or light) is moving relative to an observer. This relative motion causes a change in the observed frequency and wavelength of the wave. In terms of sound, this is often experienced when a vehicle producing sound approaches, passes and recedes from an observer.

Imagine you're standing on the side of a road and a car approaches you while honking its horn. As the car is coming towards you, the sound of the horn seems higher in pitch. This is because the sound waves are being compressed due to the motion of the car. The waves reach your ear more frequently, resulting in a higher frequency or pitch. This is known as a 'blue shift'.

However, as the car passes you and starts moving away, the pitch of the horn seems to drop. This is because the sound waves are now being stretched out, reaching your ear less frequently, resulting in a lower frequency or pitch. This is known as a 'red shift'.

The Doppler effect is not only limited to sound. It also applies to light and other electromagnetic waves. For example, astronomers use the Doppler effect to determine whether a star or galaxy is moving towards or away from Earth by analysing the light it emits. If the light is shifted towards the blue end of the spectrum, the star or galaxy is moving towards us. If it's shifted towards the red end, it's moving away.

In summary, the Doppler effect is a fascinating phenomenon that helps us understand how the frequency of waves changes with relative motion. It has significant applications in various fields, including physics, astronomy, and even medical imaging.