The Doppler shift, named after Austrian physicist Christian Doppler, refers to the change in frequency or wavelength of a wave as observed by an observer moving relative to the source of the wave. This effect is commonly observed with waves, including sound waves and light waves.
In the context of sound waves, the Doppler shift explains why the pitch of a sound appears to change when a sound source and an observer are in relative motion. The Doppler effect can be observed in everyday situations, such as when a vehicle with a siren passes by.
When a sound source is stationary or moving towards an observer, the sound waves get compressed, resulting in a higher frequency (or shorter wavelength) as perceived by the observer. This leads to a higher pitch. Conversely, if the sound source is moving away from the observer, the sound waves get stretched, resulting in a lower frequency (or longer wavelength) and a lower pitch.
For example, imagine a car with a siren moving towards you. As the car approaches, the sound waves it generates become compressed, causing an increase in frequency. Consequently, the pitch of the siren sounds higher. Once the car passes and moves away, the sound waves become stretched, resulting in a decrease in frequency and a lower pitch.
The Doppler effect also plays a role in how we hear sounds in our everyday lives. When sound waves reach our ears, they cause the eardrum to vibrate. The frequency of these vibrations corresponds to the pitch we perceive. The Doppler effect helps us discern the relative motion of sound sources, allowing us to identify the direction from which a sound is coming and whether it is approaching or receding.
In addition to sound, the Doppler effect is also applicable to other types of waves, such as light waves. It explains phenomena like the shifting of color in objects moving relative to an observer (redshift or blueshift), which has significant implications in fields like astronomy for studying the motion of celestial bodies.