Relative Motion

In Doppler effect problems involving relative motion of both the observer and the source, the frequency observed by the observer changes depending on their relative speeds. When both the source and the observer move towards each other with equal speeds, the observed frequency is affected by the relative speed between the source and the observer. The formula for the Doppler effect when both the source and observer are moving towards each other is given by: where: - is the observed frequency, - is the frequency at the source. Thus, once the source and observer cross each other, the observed frequency is given by the expression .