What effect describes an apparent shift in frequency due to relative motion between a sound or light source and an observer?

The Doppler effect is a phenomenon that occurs when there is relative motion between a source of waves (such as sound or light) and an observer. This effect causes a change in the frequency of the waves as perceived by the observer. When the source moves towards the observer, the waves are compressed, resulting in a higher frequency or pitch (this is known as a blue shift in the context of light). Conversely, when the source moves away, the waves are stretched, leading to a lower frequency or pitch (known as a red shift for light).

This effect is commonly experienced not just with sound, such as the noticeable change in pitch of a passing siren, but also in astronomy, where it helps scientists determine the motion of celestial bodies. Its significance extends beyond just sound and light; the Doppler effect applies to all types of waves under relative motion, making it a fundamental concept in wave physics.