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.
玛丽听到了救护车刺耳的警报声。
另一个例子是“警报器”这个词，是我们很熟悉的一个机械装置，当警车、救护车或消防车靠近时，警报器会发出警报。
Another example is the word siren, familiar to us as the mechanical device that makes such an alarming sound when police cars, ambulances, or fire engines approach.
它是一种类似的多普勒频移的效应，就像救护车在你面前驶过时，会促使逐渐靠近你的警报器明显地改变其音调那样。
A similar effect, called the Doppler shift, causes the siren of an approaching ambulance to apparently change pitch as it passes you.
警报声表明救护车快到了。
救护车响着警报急驰而过。
解决此问题的办法是戴一个手镯，在人生病时发出警报并叫救护车。
One way around the problem is to wear an alarm bracelet that detects when something is wrong and calls an ambulance.
伴着晚上警报器的鸣叫声，救护车把婴儿送到田纳西州诺克斯维尔市的圣玛丽医院，新生儿重病特护区。
With siren howling in the night, the ambulance rushes the infant to the neonatal intensive care unit at St. Mary's Hospital in Knoxville, Tennessee.
伴着晚上警报器的鸣叫声，救护车把婴儿送到田纳西州诺克斯维尔市的圣玛丽医院，新生儿重病特护区。
With siren howling in the night, the ambulance rushes the infant to the neonatal intensive care unit at St. Mary's Hospital in Knoxville, Tennessee.
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