Final answer:
The frequency of light decreases (redshift) when the source moves away from an observer and increases (blueshift) when it moves toward the observer, due to the Doppler effect.
Step-by-step explanation:
When a source of electromagnetic waves, such as light, moves away from an observer, the frequency of the observed light decreases, leading to what is known as a redshift. Conversely, when the source moves towards the observer, the frequency increases, causing a blueshift. This phenomenon is caused by the Doppler effect, which also applies to sound waves and is a fundamental concept in astronomy for determining the motion of distant celestial objects relative to the Earth.
A more significant difference in the motion towards or away from the observer will result in a more considerable Doppler shift. If the relative motion is entirely along the line of sight, astronomers can use a specific formula to quantify the Doppler shift in the light.