<P> f = c + 0 / c--0.7c f = 3.33 f and an observer behind the source will hear a lower frequency </P> <P> f = c + 0 / c--c f = ∞ and an observer behind the source will hear a lower frequency </P> <P> f = c + 0 / c--1.4c f = - 2.5 f and an observer behind the source will hear a lower frequency </P> <P> To understand what happens, consider the following analogy . Someone throws one ball every second at a man . Assume that balls travel with constant velocity . If the thrower is stationary, the man will receive one ball every second . However, if the thrower is moving towards the man, he will receive balls more frequently because the balls will be less spaced out . The inverse is true if the thrower is moving away from the man . So it is actually the wavelength which is affected; as a consequence, the received frequency is also affected . It may also be said that the velocity of the wave remains constant whereas wavelength changes; hence frequency also changes . </P>

The doppler shift only depends on which of the following