科学素养与现象阐释·英语30篇（5）
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Why Rainbows Typically Appear After Rainfall
为什么彩虹常出现在雨后
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Rainbows form when sunlight undergoes internal reflection and dispersion inside spherical water droplets suspended in the atmosphere.
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Post-rain conditions provide the optimal combination: abundant falling droplets and concurrent sunlight—usually from a clearing sky.
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Droplet size distribution determines rainbow sharpness: uniform 0.5–2 mm spheres yield vivid arcs; mist creates broad, washed-out glows.
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The primary bow appears at 42° from the antisolar point—requiring observer, sun, and rain to align precisely in three-dimensional space.
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Secondary rainbows form from two internal reflections, appearing fainter and reversed in color sequence at 51° elevation.
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Urban light pollution suppresses rainbow visibility, making them rarer in megacities despite frequent showers.
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Aerosol loading from pollution or wildfires scatters light, reducing contrast and often eliminating the supernumerary fringes.
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Polarization measurements confirm rainbow light is partially linearly polarized—evidence of its reflection-dominated origin.
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Mountainous terrain enhances occurrence by triggering orographic lift and localized convective showers.
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Meteorologists now use rainbow geometry in lidar calibration to validate atmospheric particle sizing algorithms.
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Its transient beauty masks a rigorous optical geometry—governed by Snell’s law and ray tracing mathematics.
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Recognizing this phenomenon cultivates attention to light–matter interaction as a daily observable physics lesson.