Final answer:
The Doppler effect is the shift in observed frequency due to the relative motion between a light source and an observer; it results in a higher perceived frequency when moving towards the source and a lower one when moving away.
Step-by-step explanation:
The phenomenon being described is known as the Doppler effect, which occurs when an observer views a moving light source and therefore perceives a different frequency of light than the one that was emitted. When the relative motion of source and observer is toward each other, the perceived frequency increases, whereas when the relative motion is apart, the perceived frequency decreases. This effect is not limited to light; it can also be observed with sound. For example, when an observer moves toward a stationary sound source, the observed frequency is higher than the source frequency, and when the observer moves away, the observed frequency is lower.
The greater the relative speed between the source and the observer, the greater the shift in perceived frequency. This concept is crucial in various fields, including astrophysics, where it helps in determining the movement of distant celestial objects, and in everyday technology such as radar and medical imaging.