Doppler Shift (doppler Effect)

The Doppler Shift, also known as the Doppler Effect, is a phenomenon that is observed when the source of a wave (whether it is a light wave, sound wave, or water wave) is moving in relation to the observer. One classic example of a Doppler Shift is when we hear the high-pitched siren of an ambulance as it moves towards us, suddenly dropping as it passes us. 

Suppose that we have a stationary ambulance, and we are standing 100 feet in front of it. Let's say that siren of the ambulance was emitting its warning sound at a frequency of one per second. Imagine each sound as a circle traveling outward from the siren. We would then have a series of concentric circles being emitted from the source, with each circle reaching us at a rate of one per second. As the ambulance starts moving towards us, it continues to emit a sound at the same frequency. But this time, since it is moving closer to us, each circle reaches us at a faster rate, or at a higher frequency, causing us to perceive the sound at a higher pitch. As soon as it passes is, the pitch suddenly lowers. This is because as the ambulance moves away from us, each circle takes longer to reach us. The sound is now moving at a lower frequency, causing us to hear a lower pitch."