STEM与日常科技·英语30篇（3）

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多普勒效应与雷达测速

When a police radar gun sends out radio waves, moving cars reflect them back at slightly changed frequencies.
If the car approaches, reflected waves bunch up—raising frequency like a siren pitch rising as an ambulance nears you.
If the car moves away, waves stretch out—lowering frequency, just as the siren drops in pitch after passing.
The radar unit measures this tiny frequency shift—often less than 0.001%—to compute exact speed in miles per hour.
This same principle helps meteorologists track storm rotation and doctors monitor blood flow in arteries.
Unlike lidar, radar works in rain or fog because radio waves penetrate moisture better than laser light.
Speed cameras combine Doppler radar with image capture—timing the shift ensures fairness, not just visual guesswork.
Every time you hear a train whistle change tone, you’re experiencing the same physics that keeps roads safer today.