Doppler Effect

To understand the Doppler effect let us imagine the following scene. You are standing beside a road and a police car with its siren turned on, drives by you. What do you notice about the sound? The siren’s sound isn’t so loud when it is at a distance, which then reaches a maximum when it is just beside you, diminishing again as it crosses and moves away from you.

Consider another instance. Two people A and B are standing on the road as shown below in the picture.

Doppler Effect

Which person do you think hears the sound of the revving engine with a greater magnitude? You know that its person A but why?

This is where we discuss the Doppler Effect or the Doppler Shift. To highlight this effect you should understand the difference between the two situations given below.

Situation 1: How are the pattern of waves formed when you suddenly jump into a pond?

Situation 2: How are the pattern of waves formed when you are walking in a pond?

The image given below highlights the difference of wave patterns in both the situations.

Doppler Effect

This difference is because the source of the waves in the second case moves. This is what the Doppler Effect is. It is named after the physicist Christian Doppler who proposed this in the 19th century. The Doppler effect is the change of frequency of a wave emitted as observed by an observer moving relative to the source. In this, the frequency received by the observer is higher during approach, identical when the relative positions are the same, and keeps lowering on recession of source. If both the source and observer are moving, the total Doppler Effect is calculated based on both these motions.

Applications of Doppler Effect

  • Sirens
  • Radar
  • Astronomy
  • Medical Imaging
  • Blood Flow Measurement
  • Satellite Communication
  • Vibration Measurement
  • Developmental Biology
  • Audio
  • Velocity Profile Measurement

Practise This Question

A train has arrived into a track which is a semicircle. At an instant when the engine is at the forward end of the semi circular part of the track while the last carriage is at the rear end, the driver blows a whistle of frequency 200 Hz. Velocity of sound is 340 m/sec. Then the apparent frequency as observed by a passenger in the middle of the train at the same instant is, (the speed of the train is 30 m/second)