The Doppler effect is a phenomenon that occurs when a sound or light source moves relative to an observer. When the source approaches, the waves compress, resulting in a higher frequency or pitch. Conversely, as the source moves away, the waves stretch, leading to a lower frequency or pitch. This effect is commonly experienced with ambulance sirens or passing trains. In addition to sound, the Doppler effect also applies to light. When a star or galaxy moves away from Earth, its light shifts to longer wavelengths, known as redshift. If it moves closer, the light shifts to shorter wavelengths, called blueshift. This effect helps astronomers understand the movement of celestial objects.