Answer to Question #214980 in Atomic and Nuclear Physics for Iron

Question

A police officer's stationary radar device indicates that the frequency of the radar wave reflected from automobile is less than the frequency emitted by the radar device. This indicates the automobile is?

Answer 1

The automobile is moving away from the police officer.

Reason: Due to the direct consequence of Doppler's effect .

Other words:-

This occur due to Doppler effect.

An easy way to understand this is to think in terms of wavelength.

think like this ,suppose the automobile is moving towards the officer,

the wave in between the officer and automobile seems to be a compressed,so wavelength decreases so frequency increases and vice versa. although this is not how the variation in wavelengtht takes place it is still helpful in figuring out the variation with frequency.