根据多普勒效应，光被离子反射后，靠近激光器的光的频率会高于远离激光器的光的频率，（日常生活中，救护车警报器的音调随着车的远近也变化，就是这个道理）这才有可能测量先前对离子施加的力的大小。

(An everyday example of this effect is the change of pitch of an ambulance siren as it approaches and recedes.) It was thus possible to measure the force applied to the ions.

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