Imagine, a boat moving on the placid waters of a lake. Initially, small waves will appear as the boat starts; big waves will build up as the boat pick up; the waves will overlap and superimpose on each other as the boat gains more and more speed; V shaped wave formation appears. It is what we call "wake"(also shock wave) . Here the point to note is "moving boat compresses the water waves".
The pitch of the siren of the speeding ambulance changes as it approaches us or recedes from us. It is because the sound waves are compressed, (frequency increasing) in front of the vehicle and spread out (decrease of frequency) at the rear side of the vehicle. This is called "Doppler effect".
The statement: Whenever there is a relative motion between the source of waves and the observer, the frequency of waves changes.
An important discovery in physics.
The sound generated from the airplane engine normally travels faster than the plane. (330 m/s). A ordinary air plane cannot catch up with the sound waves. But the supersonic plane can fly faster than the sound waves. So the sound waves condenses and the plane carry "wake" (mach cone here) of sound waves. whenever the supersonic plane flies over our head, we hear the "wake" as a boom. In the case of water waves, we see the V shaped wave bunch. In the case of sound waves, we hear cone shaped wave bunch. .
Doppler effect is also applicable to light waves. We know sunlight or starlight can give out spectrum of colours. A star may move towards us or away from us. Consequently the star light frequency may change and there will shift in the positions of the colours of star spectrum. We call it Red shift or Blue shift using which the velocity and the direction of the star can be calculated.
If you speed up your car beyond the limit, an officer can detect the car speed with his electronic device using doppler effect.
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