What phenomenon is described by the Doppler effect?

The Doppler effect describes the change in frequency or wavelength of a wave in relation to an observer who is moving relative to the wave source. This phenomenon occurs in various types of waves, including sound and electromagnetic waves.

When the source of the wave moves towards an observer, the waves are compressed, leading to a higher frequency (or shorter wavelength), which can be perceived as a higher pitch in sound, for instance. Conversely, when the source moves away from the observer, the waves are stretched, resulting in a lower frequency (or longer wavelength), perceived as a lower pitch. This effect is commonly experienced with passing sirens or vehicles, where the sound changes as they approach and then recede.

Understanding the Doppler effect is crucial in various fields, including astronomy, where it helps determine the speed and direction of stars and galaxies based on the changes in the wavelength of light they emit.