STEM与日常科技·英语30篇（3）
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Doppler Effect and Radar Speed Detection
多普勒效应与雷达测速
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When a police radar gun sends out radio waves, moving cars reflect them back at slightly changed frequencies.
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If the car approaches, reflected waves bunch up—raising frequency like a siren pitch rising as an ambulance nears you.
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If the car moves away, waves stretch out—lowering frequency, just as the siren drops in pitch after passing.
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The radar unit measures this tiny frequency shift—often less than 0.001%—to compute exact speed in miles per hour.
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This same principle helps meteorologists track storm rotation and doctors monitor blood flow in arteries.
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Unlike lidar, radar works in rain or fog because radio waves penetrate moisture better than laser light.
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Speed cameras combine Doppler radar with image capture—timing the shift ensures fairness, not just visual guesswork.
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Every time you hear a train whistle change tone, you’re experiencing the same physics that keeps roads safer today.