What is the Doppler Effect?


wiseGEEK Writing Contest

In 1815 a British engineer by the name of George Stephenson invented the steam locomotive. Soon there was a vehicle moving faster than a horse could gallop and since the trains were fitted with whistles that tooted when approaching a crossing or station, it was noticed that the sound emitted from the whistle would change pitch. It seemed higher in pitch on the approach, seemed normal when abreast of the listener and as it receded in the distance it became lower. It was a puzzle until the Austrian physicist, Christian Johan Doppler decided to tackle the problem. He studied the problem by having a locomotive pull a flat car back and forth at different speeds to determine if there was any difference. He knew that sound traveled in waves and correctly surmised that as the source of the sound was approaching the listener, the velocity of the source sped up the sound waves shortening the wave length and making the pitch higher. As the source passed the listener, the sound was true but after it passed, the wave lengths of the sound were stretched causing the pitch to lower. He was able to work out a mathematical relationship to speed and pitch and the phenomenon is called the Doppler Effect.

At this time, it was also known that light traveled in waves. In 1858 a French physicist by the name of Armand Fizeau suggested that the Doppler Effect should work with any wave motion, including light. Using this knowledge, astronomers could now determine whether a star or galaxy was moving toward our solar system or moving away and at what speed. When light is passed through a spectroscope it produces vertical lines. These lines represent the waves of light coming into the device. Consistent with the Doppler Effect, if the source of light is moving away from the Earth, the wavelengths are longer which will shift the lines to the red end of the spectrum. This is called a red shift and conversely, if the source of light is moving closer, the wavelengths are shorter (higher pitch) and the spectral lines move toward the violet end of the spectrum. This motion of the heavenly bodies is called radial motion since the apparent movement is on a line radiating outward from our vantage point, the planet Earth. An astronomer by the name of William Huggins was the first to determine the radial velocity of a star. He was close at 46 kilometers per second. A good result for 1868.

submitted by philip a baker