Explained: What is Doppler Effect in Physics?

Wave frequency is a science concept that students often find difficult to understand. The Doppler Effect is a motion-related wave frequency change phenomena in Physics. When there is a change in the frequency of a wave as the source/origin of that wave and the observer/ person move towards or away from each other is known as Doppler Effect. The concept derives its name from Austrian physicist, Christian Doppler who described the phenomena in 1842.  

The Doppler Effect, states, “The apparent difference between the frequency at which waves leave a source and the frequency at which they reach an observer, produced by relative motion between the observer and the wave source is known as Doppler Effect or Doppler Shift.” 

Real-life example of Doppler Effect 

The Doppler Effect can be observed in a real-life example as explained below. 

Imagine that a bug resides in the centre of a circular water puddle. The bug shakes every now and again. To create disturbances that pass across the water, it uses its legs. If the disturbances begin at a single spot, they will spread outward in all directions from that point. Since all of the disturbances are going through the same medium, they will all travel at the same speed in all directions. A sequence of concentric rings would be generated by the bug's shaking. At the same time, these rings would reach the margins of the water puddle. The disturbances to strike the puddle's edge would be detected by an observer on the left edge of the puddle at the same frequency as an observer on the right edge of it. In reality, the frequency with which disturbances reach the puddle's edge corresponds to the frequency with which the bug causes the disturbances. If the bug creates disturbances at a rate of 2 per second, each observer will see them arriving at a rate of 2 per second.  

Suppose that the bug is now travelling to the right across the puddle of water, creating two disturbances every second. Because the bug is travelling to the right, each subsequent disturbance comes from a location closer to the right-hand observer and farther away from the left-hand observer. Following that, each successive disturbance must travel a shorter distance before reaching the observer on the right side. As a result, it takes less time to reach the observer on the right side, who notices that the frequency of disturbance arrival is higher than the frequency of disturbance production.  

Each successive disturbance, on the other hand, must travel a greater distance before reaching the observer on the left. As a result, he or she notices a lower frequency of arrival than the frequency at which the disturbances occur. The total impact of the bug's motion (the source of waves) is that the observer facing the insect sees a frequency more than two disturbances per second, while the observer facing away from the bug sees a frequency less than two disturbances per second. This is the Doppler Effect being observed whenever the source of waves is moving with respect to an observer. 

Applications of Doppler Effect 
The Doppler Effect is used in multiple fields such as sirens, radars, medical imaging and blood flow management, astronomy, satellite communication, vibration measurement and velocity profile management.