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 波長
Spectrum 光譜
光的波長（或者相鄰波峰之間的距離）極其微小，約為0.0000004至0.0000008米。光的不同波長正是人眼看到的不同顏色，最長的波長出現在光譜的紅端，而最短的波長在光譜的藍端。想像在離開我們一個固定的距離處有一光源——例如恆星——以固定的波長髮出光波。顯然我們接收到的波長和發射時的波長一樣（星系的引力場沒有強到足以對它產生明顯的效應）。
現在假定這恆星光源開始向我們運動。當光源發出第二個波峰時，它離開我們更近一些，這樣兩個波峰之間的距離比恆星靜止時更小。這意味著，我們接收到的波的波長比恆星靜止時更短。相應地，如果光源離開我們運動，我們接收的波的波長將更長。這意味著，當恆星離開我們而去時，它們的光譜向紅端移動（紅移）；而當恆星趨近我們而來時，光譜則藍移。這個稱之為多普勒效應的頻率和速度的關係是我們日常所熟悉的，例如我們聽路上來往小汽車的聲音：當它開過來時，它的發動機的音調變高（對應於聲波的高頻率）；當它通過我們身邊而離開時，它的音調變低。
光波或無線電波的行為與之類似。警察就是利用多普勒效應的原理，以無線電波脈衝從車上反射回來的頻率來測量車速。
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