17.15:
Doppler Effect – II
The Doppler effect is described as the change in the observed frequency of the sound waves, whenever the source of waves is moving with respect to an observer.
For instance, when a fire truck approaches a man, the sound waves that are emitted bunch up, creating an upward shift in frequency, which is interpreted as a high pitch.
However, when the fire truck recedes away, the waves spread out and the man observes an apparent downward shift in frequency, which is interpreted as low pitch.
In another case, when a man approaches a stationary fire truck, he strikes more wave crests, resulting in high-frequency sound. Conversely, when the man moves away, he strikes fewer wave crests, resulting in a lower frequency of sound.
Thus, the Doppler effect can be observed for two cases: first, when the source moves towards or away from the stationary observer, and second, when the observer moves towards or away from the stationary source.
17.15:
Doppler Effect – II
The Doppler effect has several practical, real-world applications. For instance, meteorologists use Doppler radars to interpret weather events based on the Doppler effect. Typically, a transmitter emits radio waves at a specific frequency toward the sky from a weather station. The radio waves bounce off the clouds and precipitation and travel back to the weather station. The radio frequency of the waves reflected back to the station appears to decrease if the clouds or precipitation are moving away from the station, whereas they increase if moving toward the station. This technology can also be applied to determine wind speeds and direction.
Similarly, the Doppler effect has applications in medical physics. In a Doppler echocardiogram, sound waves of a certain frequency are channeled into the heart and reflect off of blood cells moving through the heart and blood vessels. Like Doppler radar, cardiologists can understand the speed and direction of blood flow in the heart due to the shift in frequencies received after reflection. This can help them identify areas of obstruction in the heart.
This text is adapted from Openstax, University Physics Volume 1, Section 17.7: The Doppler Effect.