Let’s start with a simple example in which a bug is at the centre of curve water puddle. It travels with producing vibration in the water by its legs. This disturbance is also travelled from a point in all directions and bug is moved in concentric circle. Let there are two observers on both sides of concentric circle. The frequency of disturbance at both the points is same as the frequency of bug’s disturbance. But if the bug is moving towards one observer at right side and producing disturbance near to that observer and far to the opposite side of the observer. Thus each disturbance travels with smaller distance and less time to reach that specific observer to right side. This time the frequency of arrival disturbance is greater than the frequency of produced disturbance.
