FTP, name this physical phenomenon that causes the pitch of the siren on a passing police car to change relative to a stationary observer.
The proximity fuse, developed during World War II, applies this principle to detonate automatically when the target is close enough. Since light waves do not travel through a medium, they require the relativistic form of this, which takes into account time dilation. The classical form was tested and confirmed for sound (*) waves in 1845 by Dutch scientist C.H.D. Buys Ballot.  For electromagnetic waves, it results in redshift. It relates the transmitted and observed frequencies of a wave in terms of its velocity and the velocity of its source. FTP, name this physical phenomenon that causes the pitch of the siren on a passing police car to change relative to a stationary observer.
The proximity fuse, developed during World War II, applies this principle to detonate automatically when the target is close enough.
{Doppler} effect or {shift}