Doppler Effect
The Doppler Effect describes the change in the frequency of a wave when the source and the observer move relative to each other. Christian Doppler first explained the phenomenon in the 19th century. Sound waves and light waves both show this effect. As a result, the pitch of a sound or the color of light can shift depending on motion.
A familiar example occurs with an approaching ambulance. The siren produces a higher-pitched sound as the vehicle moves toward a listener. The waves compress in front of the moving source. After the ambulance passes, the pitch drops. The waves stretch out behind the source. Therefore, the observer hears a clear change in frequency even though the siren itself remains constant.
The same principle applies to light. When a star or galaxy moves toward Earth, its light shifts toward the blue end of the spectrum. When it moves away, the light shifts toward red. Astronomers call these changes blueshift and redshift. Moreover, the amount of the shift reveals the speed of the distant object. This measurement helps scientists map the expansion of the universe.
Radar systems use the Doppler Effect to measure speed. A radar gun sends radio waves toward a moving vehicle. The waves bounce back at a different frequency. The device calculates the speed from the size of the frequency change. In addition, weather radar detects the motion of raindrops and wind. Meteorologists then track storms and issue warnings.
Medical ultrasound also relies on the Doppler Effect. Sound waves bounce off moving blood cells. The returning waves change frequency according to the speed and direction of blood flow. Doctors use this information to check heart valves and blood vessels. The technique remains non-invasive and provides real-time results.
The Doppler Effect therefore appears in everyday life and in advanced scientific tools. Relative motion changes the observed frequency of waves. Understanding this shift allows accurate measurements of speed, distance, and motion across many fields.