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## Physics Tutorials

### The Doppler Effect

Doppler Effect: The frequency of the sound waves produced by a source can be changed by the motion of the source, by the observer, or both at the same time. Doppler effect is experienced as an increase in observed frequency as an object moves toward the observer and a decrease in frequency as the object moves away.

#### 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.

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 away from the earth, then the frequency of the light emitted by it will decrease and will be shifted to the red end of the spectrum because the red end of the visible spectrum has a lower frequency.

#### Question

How does radar determine the speed of a

a) stationary car

b. moving car?