What is the Doppler Effect

Doppler Effect is the apparent change in the frequency of a wave when the source producing the wave moves relative to the observer. The phenomenon derives its name from physicist, Christian Doppler who first explained by in the year 1842. The most common experience with Doppler Effect is felt when a source of sound approaches a stationary person. It is observed that as the source producing the sound waves comes close to the person, there appears to be an apparent increase in the frequency. The frequency of the sound waves seems to decrease as the source moves away from the listener.

The reason for this is given as follows. As a source producing the wave nears the observer, the point at which the waves are produced starts nearing the observer. Thus, each time a new wave is emitted, there seems to be an apparent increase in the frequency as each wave reaches the observer faster than the previous wave. Similar explanation holds good for the decrease in frequency that is felt by the observer as the source producing the wave moves away from him. This phenomenon is applicable to waves of all kinds including sound waves, water waves and light waves.

The application of the phenomenon of Doppler Effect is seen in many fields. In the field of astronomy, the concepts of red shift and blue shift that are used to measure the speed at which stars approach or recede work on the principle of Doppler Effect. Red shift refers to the decrease in frequency of the light and therefore, represents a receding star and blue shift signifies increase in frequency, indicating the approaching star. RADAR used for military purposes too works on the principle of Doppler Effect and helps in detecting encroachment of any kind by the enemy planes.

In the field of fluid mechanics, with the use of Doppler Effect, the devices, LDV (laser Doppler velocimeter) and ADV (acoustic Doppler velocimeter) are used to measure the speed of fluids. Doppler Effect finds application in the field of medicine too, wherein irregularities of any kind in the flow of blood is detected by echocardiogram. Doppler RADAR is used in weather forecasting.

One should note the point that the increase or decrease in frequency that is noticed is apparent and in reality, there is no change in the frequency of the source. The large number of devices that work on the principle of Doppler Effect goes on to reflect the revolution that has been brought about in the field of technology owing to the discovery of Doppler Effect.

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