The wavelength (or distance from one wave crest to the next)of light is extremely small, ranging from four to seven ten-millionths of a meter. The different wavelengths of light are what the human eye sees as different colors, with the longest wavelengths appearing at the red end of the spectrum and the shortest wavelengths at the blue end. Now imagine a source of light at a constant distance from us, such as a star, emitting waves of light at a constant wavelength. Obviously the wave-length of the waves we receive will be the same as the wavelength at which they are emitted (the gravitational field of the galaxy will not be large enough to have a significant effect).
Suppose now that the source starts moving toward us. When the source emits the next wave crest it will be nearer to us, so the distance between wave crests will be smaller than when the star was stationary. This means that the wavelength of the waves we receive is shorter than when the star was stationary.
Correspondingly, if the source is moving away from us, the wavelength of the waves we receive will be longer. In the case of light, therefore, means that stars moving away from us will have their spectra shifted toward the red end of the spectrum(red-shifted) and those moving toward us will have their spectra blue-shifted. This relationship between wavelength and speed, which is called the Doppler effect, is an everyday experience. Listen to a car passing on the road: as the car is approaching, its engine sounds at a higher pitch (corresponding to a shorter wavelength and higher frequency of sound waves),and when it passes and goes away, it sounds at a lower pitch.
The behavior of light or radio waves is similar. Indeed, the police make use of the Doppler effect to measure the speed of cars by measuring the wavelength of pulses of radio waves reflected off them.
Wavelength 波长
Spectrum 光谱
光的波长（或者相邻波峰之间的距离）极其微小，约为0.0000004至0.0000008米。光的不同波长正是人眼看到的不同颜色，最长的波长出现在光谱的红端，而最短的波长在光谱的蓝端。想像在离开我们一个固定的距离处有一光源——例如恒星——以固定的波长发出光波。显然我们接收到的波长和发射时的波长一样（星系的引力场没有强到足以对它产生明显的效应）。
现在假定这恒星光源开始向我们运动。当光源发出第二个波峰时，它离开我们更近一些，这样两个波峰之间的距离比恒星静止时更小。这意味着，我们接收到的波的波长比恒星静止时更短。相应地，如果光源离开我们运动，我们接收的波的波长将更长。这意味着，当恒星离开我们而去时，它们的光谱向红端移动（红移）；而当恒星趋近我们而来时，光谱则蓝移。这个称之为多普勒效应的频率和速度的关系是我们日常所熟悉的，例如我们听路上来往小汽车的声音：当它开过来时，它的发动机的音调变高（对应于声波的高频率）；当它通过我们身边而离开时，它的音调变低。
光波或无线电波的行为与之类似。警察就是利用多普勒效应的原理，以无线电波脉冲从车上反射回来的频率来测量车速。
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