Which principle states that the frequency of radiation observed changes based on the relative velocity of the source?

The correct answer is the Doppler Effect, which is a fundamental principle describing how the observed frequency of a wave changes when there is relative motion between the source of the wave and the observer. When a sound source moves towards an observer, the waves are compressed, leading to a higher frequency being observed, which is perceived as a higher pitch. Conversely, when the source moves away, the waves are stretched, resulting in a lower frequency and perceived lower pitch.

This concept is applicable not only to sound waves but also to electromagnetic waves, such as light. When a light source moves towards an observer, its light is perceived at a higher frequency (blueshift), whereas if it's moving away, the light is perceived at a lower frequency (redshift). The Doppler Effect is widely used in astronomy to determine the motion of stars and galaxies based on their light spectra.

The other choices do not accurately capture the comprehensive nature of this phenomenon. While frequency shift, wavelength change, and redshift principle all relate to aspects of wave behavior, the Doppler Effect specifically encompasses the entire principle regarding the shifts in frequency due to movement, making it the correct response to this question.