On a day in 1842, Austrian physicist Doppler noticed an interesting phenomenon while passing through a railway crossing: although the frequency of the train's whistle was fixed, he heard it sounding sharper when the train was approaching and lower when it was leaving. This phenomenon, in which the observed frequency of a sound source differs from the actual frequency due to the relative motion between the source and the observer, is known as the Doppler effect or frequency shift phenomenon.
The Doppler effect is easy to understand. When the train is stationary, there is no relative motion between it and the passengers on the platform, so the passengers hear the whistle at its original pitch. When the train approaches, the front sound waves are compressed, causing the wavelength to shorten and the frequency to increase, making the whistle sound sharper to the passengers. When the train leaves, the rear sound waves have more space to travel, causing the wavelength to lengthen and the frequency to decrease, making the whistle sound lower to the passengers. The faster the relative motion of the sound source to the observer, the more pronounced the pitch change will be. The Mach number, which is the ratio of an object's speed to the speed of sound, is an important parameter determining the strength of the Doppler effect. The larger the Mach number, the more pronounced the Doppler effect will be. When the object's speed equals the speed of sound, the observer will no longer be able to hear the sound before the object reaches them. Ultrasound and light (electromagnetic waves) also exhibit the Doppler effect.
The Doppler effect has many applications in different scenarios. Doppler radar guns are one of them. Highways are equipped with monitors equipped with Doppler radar guns. The radar gun emits ultrasound waves of a known frequency toward a moving vehicle and measures the frequency of the reflected waves. Based on the change in the reflected wave's frequency, the vehicle's speed can be accurately determined. If a driver receives a ticket for speeding on the highway, it means they have experienced an accurate speed measurement based on the Doppler effect.
Doctors can also use the Doppler effect between ultrasound sources and moving blood to check the flow speed of blood in the heart and blood vessels. When the blood vessel moves toward the ultrasound source, the reflected wave's wavelength is compressed, causing the frequency to increase; when the blood vessel moves away from the ultrasound source, the reflected wave's wavelength lengthens, causing the frequency to decrease. Based on the amount of frequency shift received from the ultrasound wave, the blood's flow speed can be measured.