What is Doppler effect?

Doppler effect is a phenomenon that occurs when there is a change in the frequency or wavelength of sound or light waves as a result of the relative motion between the source of the waves and the observer. It was first observed by Christian Doppler, a physicist from Austria, in 1842 and since then, it has been used in various fields such as astronomy, medicine, and radar technology.

Understanding the physics behind it

The Doppler effect is caused by the change in distance between the source of the waves and the observer. If the source of the waves is moving towards the observer, the frequency of the waves increases and their wavelength decreases. Conversely, if the source of the waves is moving away from the observer, the frequency of the waves decreases and their wavelength increases.

The Doppler effect can be expressed by the equation:

f’ = f (v + vr) / (v – vs)

Where:
f’ is the observed frequency
f is the frequency of the source
v is the velocity of the waves in the medium
vr is the velocity of the observer relative to the medium
vs is the velocity of the source relative to the medium

Applications of Doppler effect

The Doppler effect has many practical applications in various fields:

• In medicine, it is used in ultrasound to detect the movement of blood cells and hence diagnose conditions such as heart disease.
• In astronomy, it is used to determine the speed and direction of celestial objects such as stars and galaxies.
• In weather forecasting, it is used to detect the speed and direction of winds by measuring the frequency of the waves reflected by the moving air molecules.
• In police radar guns, it is used to measure the speed of moving vehicles by measuring the frequency shift of the radar waves reflected by the vehicles.

Examples of Doppler effect in everyday life

The Doppler effect is present in many situations we encounter in our daily lives:

• The sound of a police siren changes as the car approaches and passes us. This is because the frequency of the sound waves increases as the car approaches and decreases as it moves away.
• The red shift and blue shift observed in the light emitted by stars are caused by the Doppler effect. This enables us to determine the speed and distance of these stars.
• The pitch of a train whistle changes as it passes us. This is because the frequency of the sound waves changes due to the relative motion between the train and the observer.