First of all plzz correct , it is Doppler not Dobbler. Its definition ------> The frequency of a wave like signal such as sound or light depends on the movement of the sender and of the receiver. This is known as the Doppler effect. Or increase or decrease in the frequency of these waves from the sender to the receiver is known as Doppler effect.
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The Doppler effect causes the received frequency of a source (how it is perceived when it gets to its destination) to differ from the sent frequency if there is motion that is increasing or decreasing the distance between the source and the receiver.