In the early 19th century Christian Doppler, an Austrian physicist, proposed a theory regarding the properties of a moving source of sound. This is now known as Doppler’s law or the Doppler effect.

In the early 19th century Christian Doppler, an Austrian physicist, proposed a theory regarding the properties of a moving source of sound. This is now known as Doppler’s law or the Doppler effect. What does this law state?