Define Doppler effect.

The Doppler effect is the change in frequency or wavelength of a wave in relation to an observer who is moving relative to the wave source. This effect is commonly observed with sound waves, where the pitch of a sound appears to change as the source of the sound moves closer or farther away from the observer.

The Doppler effect is the change in frequency of a wave in relation to an observer who is moving relative to the wave source. It is named after the Austrian physicist Christian Doppler, who first described the effect in 1842.

The Doppler effect occurs when there is a relative motion between the wave source and the observer. If the source is moving towards the observer, the waves will be compressed, and the frequency will increase. If the source is moving away from the observer, the waves will be stretched, and the frequency will decrease.

The amount of frequency change depends on the speed of the relative motion and the wavelength of the wave. The faster the relative motion, the greater the frequency change. The shorter the wavelength, the greater the frequency change.

The Doppler effect is used in a variety of applications, including radar, sonar, and medical imaging. In radar, the Doppler effect is used to measure the speed of moving objects. In sonar, the Doppler effect is used to detect and track underwater objects. In medical imaging, the Doppler effect is used to measure blood flow.