|
The Doppler Effect is the phenomenon that occurs when the source and observer move with respect to each other so that the resulting sound wave is substantially affected in terms of frequency and velocity. This effect is observed with all types of waves. In sound waves it is very prominent. It is basically explained by realizing the effect produced by a moving source of waves in which there is an apparent upward shift in frequency for the observer and the source are approaching and an apparent downward shift in frequency when the observer and the source is receding.
An interesting example of the Doppler Effect can be when the police car passes an observer. In this case the pitch of the siren of the police car is realized to be high when the vehicle approaches the observer and the pitch is realized to be low when the vehicle already crossed the observer. This is shown as below:
Another interesting example is the apparent change in frequency of the train horn. As the train approaches, the sound of its horn is heard at a high pitch and as the train moved away, the sound of its horn is heard at a low pitch.
From the above examples, it is realized that the Doppler Effect is observed because of the change in distance between the source of sound and the observer. If the source and the observer are approaching, then the distance is decreasing and if the source and the observer are receding, then the distance is increasing. The source of sound always emits the same frequency. If the distance is large, then the waves can be spread apart; but if the distance is small, the waves must be compressed into the smaller distance. Thus, if the source is moving towards the observer, the observer perceives sound waves at a more frequent rate or with high pitch. And if the source is moving away from the observer, the observer perceives sound waves at a less frequent rate or low pitch. Hence, the Doppler Effect is a shift in the apparent or observed frequency and not a shift in the actual frequency at which the source vibrates.
If an observer moves with a speed v0 relative to a stationary source, the frequency of sound heard by the observer is given by,
Here, f is the frequency of the source and v is the speed of sound. When the observed is moving towards then +ve sign is considered and when it is moving away then –ve sign is considered.
If a source is in motion with a speed vs relative to the stationary observer, then the frequency of sound wave heard by the observer is given by,
When both source and observer both are in motion, the general relationship for the observed frequency is given as:
Here, the upper signs refer to motion of one towards the other, and the lower signs refer to motion of one away from the other. And, the word ‘toward’ is associated with an increase in observed frequency. The words ‘away from’ are associated with a decrease in observed frequency.
|
Waves and Optics