Doppler Effect
The Doppler Effect occurs when there is relative motion between a wave source and an observer. As a result, the wavelength and frequency of the waves detected by the observer differ from their actual values.
When the source is moving away from the observer, the waves are stretched. This means the observed wavelength increases, and the frequency decreases compared to the actual values.
When the source is moving towards the observer, the waves are compressed. The observed wavelength becomes shorter, and the frequency increases.
The greater the relative speed between the source and the observer, the greater the change in wavelength and frequency.