What is the Doppler Effect?
When fire sirens or police cars are rushing somewhere you can hear them approach. They do not have to follow normal traffic rules and can run past traffic signals. Since they must reach the scene without any delay, they use their sirens to tell people to move out of the way. Sirens on vehicles are normally two long notes that continuous move from one frequency to the next. Have you noticed that these two notes seem farther apart when the source is far away and faster when the source is next to you?
This is because sound travels in waves. When the source is stationary the waves will reach you at the intervals in which they are produced. Imagine an insect sitting in the middle of a pond. The waves created from the disturbance of the insect will reach the shore uniformly. Now imagine that the insect is moving left while creating the same disturbance. Now since the bug is moving, each disturbance is being created from a different location.
The effect of the sound getting quicker and louder as the source approaches you is known as the Doppler effect. It is not caused because the beat is changing or the volume getting louder. The frequency of the waves reaching you get higher as the source approaches, it is equal at the instant it is right next to you, and lower after it passes.
People are so in tune with the sirens of police cars and fire trucks that they know when to slow down or move out of the way when one approaches. Even though they have their siren to warn people, these rescuers still slow down at junctions and stop if necessary, even though they have the right of way.
THINGS TO DO
Find a long space. It could be indoors or outside in your colony. Ask a friend to make the sound of a siren and run towards you. Do you notice the Doppler effect? Now do the same for your friend.
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