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多普勒效应- -

                                      

当振动着的波源逐渐靠近观测者时，测量到的频率比从波源发出的频率高。当波源离去时，测量到的频率则低于发出的频率。这就叫做多普勒效应或叫多普勒原理。这是以19世纪前半期的一个奥地利物理学家的名字命名的。当鸣着汽笛的机车（或其经声源）向一个静止的状态的时候更为稿密。因为音调的高低是由频率（每秒振动的次数）决定的，所以开过来的机车汽笛的音调就比机车和观测者相对静止时间听起来就越变越低了。当路上鸣着喇叭的汽车从我们身旁迅速开过，或是观测者迅速移向静止声源时（例如：一个摩托车手驶向一个警报器）也可以观测到同样的效应。     多普勒效应在天文学上广泛应用于测量距离遥远的恒星或星云靠近我们或离开我们速度。这些运动使其谱线位置产生位移。某特定的谱线就代表一个确定光的波长。如果发某种光的恒星离开我们，根据多普勒原理，其光线的波长较长（即频率较低），在光谱上则显示出谱线全部移向红端。这就称为“红移”。同样，在向我们靠近的恒星的光谱上，特征谱线会显示“蓝移”，即谱线移向蓝端，就是说波长短而频率高。

When a vibrating source of waves is approaching an observer, the frequency observed is higher than the frequency emitted by is lower than that emitted. This is known as the Doppler effect, or Doppler’s principle, and is named after an Austrian physicist who lived in the first half of the 19th century. When a whistling locomotive (or any other sound source) approaches a stationary observer, more density concentrations reach his ear than when both the sound source and the observer are stationary. As the pitch depends on the frequency (number of vibrations per second), the sound from the approaching locomotive’s whistle has a higher pitch than the sound coming from the same whistle when the locomotive is stationary in relation to the observer. Similarly, when the locomotive passes the observer, the note of the whistle is heard to change to a lower pitch. The same effect is observed when we are passed by a fast-moving hooting car in the street, or when the observer is moving fast in relation to a stationary sound source (e.g., a motor cyclist approaching a siren).         The Doppler effect is widely used in astronomy for measuring the velocity at which distant stars or nebulae are approaching or receding. These motions produce a shift in the position of lines in their spectra. A particular spectrum line corresponds to a certain definite light wave length. If the star emitting the light is moving away from us, its light rays have a longer wavelength (lower frequency) by virtue of Doppler principle and this is manifested in a general shift of the spectrum lines towards the red end of the spectrum. This is known as the “red shift”. Similarly, in the spectrum of a star moving towards us, the characteristics lines would show a “blue shift”, i.e., they would be displaced towards the blue end of the spectrum, corresponding to shorter wavelengths and higher frequencies.

- 作者： 翰唐 访问统计：　2005年05月22日, 星期日 13:18 加入博采