Who discovered that relative motion causes a signal's frequency to change?

The correct answer is Christian Doppler. He is renowned for formulating the Doppler Effect, which describes how the frequency of a wave changes for an observer moving relative to the source of the wave. This phenomenon explains that when a source of sound or light is moving toward an observer, the waves are compressed, resulting in a higher frequency or pitch. Conversely, if the source is moving away, the waves are stretched, leading to a lower frequency. Doppler's pioneering work in this area laid the foundation for various applications, including radar technology, astronomy, and even medical imaging.

The contributions of figures like Isaac Newton primarily revolved around classical mechanics and optics rather than the specific idea of relative motion affecting wave frequencies. Albert Einstein's theories of relativity also encompass the principles of motion and light but do not directly relate to the specific discovery of frequency variation due to relative motion. Galileo Galilei made significant advancements in observational astronomy and the scientific method, but he is not directly associated with the principles of wave frequency changes described by the Doppler Effect.