Who is known for discovering that relative motion causes a signal's frequency to change?

The understanding that relative motion affects the frequency of signals is primarily attributed to Christian Johann Doppler. He proposed the Doppler Effect in 1842, which describes how the frequency of a wave changes for an observer moving relative to the source of the wave. This principle is widely applicable in various fields, including astronomy, radar technology, and medical imaging, among others. For instance, it explains why an ambulance with a siren sounds different when it approaches compared to when it's moving away. The effect results from the compression of waves as the source moves closer (which raises the frequency) and their elongation as it moves away (which lowers the frequency). This concept is vital for understanding how radar systems detect speed and distance based on frequency changes of returned signals. Understanding the Doppler Effect is crucial for anyone operating radar technology, as it directly relates to the tracking and measurement of objects in motion.