Final answer:
The Doppler Effect is the change in wavelength and frequency of waves when the source is moving relative to an observer, leading to a blueshift when approaching and a redshift when receding.
Step-by-step explanation:
The change in wavelength (and, therefore, frequency) of waves that occurs when the source of the waves is moving is referred to as the Doppler Effect. This phenomenon can be observed when a moving source of sound or light waves emits waves that appear to have a shorter wavelength and higher frequency to observers moving towards the source, and a longer wavelength and lower frequency to observers moving away from the source. This is because the speed of sound or light in a medium is fixed; thus, the product of the frequency and wavelength must remain constant. As the source moves towards an observer, wave crests are encountered more frequently, leading to a higher perceived frequency or a blueshift. Conversely, as the source moves away, wave crests are encountered less frequently, resulting in a lower perceived frequency or a redshift. The Doppler Effect has practical applications in astronomy, where it helps measure the movement of stars and galaxies through the redshift and blueshift of light.