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Physics Tutorial: The Doppler Effect

Doppler Effect: The frequency of the sound waves produced by the same source can be changed by the motion of the source, or even by the observer or both at the same time.

Explanation: This effect can be explained the help of an example of a train. When a train approaches us the frequency of the whistle (or shrillness) increases, and when it goes away it decreases. This change of the frequency of sound of the moving object is called the Doppler Effect.

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The Doppler effect benefits us every day.

We can apply the Doppler Effect to light waves emitted by a star in a galaxy. We can find out whether the star is moving away or coming close to the earth. If a star is moving towards the earth, then the frequency of the light emitted by it will increase and will be shifted to the violet end of the spectrum because the violet end of the visible spectrum has a higher frequency.

Question: How does radar determine the speed of a

a.) stationary car

b.) moving car?

Answer:

a.) Speed of an object can be measured with a radar with the help of the Doppler Effect. Radio waves have a certain frequency. When the radar gun and the car are both standing still, each part of the signal is reflected when it hits the car, mirroring the original signal exactly.

b.) But when the car is moving, each part of the radio signal is reflected at a different point, because the car is in motion. When the car is moving away from the radar gun, the second part of the signal has to travel a greater distance to reach the car than the first part of the signal. If the car is moving toward the radar gun, the second part of the wave travels a shorter distance than the first part before being reflected back.

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Mohit Ghai