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Why Rainbows Typically Appear After Rainfall
为什么彩虹常出现在雨后
精读先不看文字，完整听一遍。
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Rainbows form when sunlight undergoes internal reflection and dispersion inside spherical water droplets suspended in the atmosphere.彩虹形成于阳光在大气中悬浮的球形水滴内发生内反射和色散时。
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Post-rain conditions provide the optimal combination: abundant falling droplets and concurrent sunlight—usually from a clearing sky.雨后条件最理想：大量下落的水滴与同时出现的阳光（通常来自逐渐放晴的天空）共同作用。
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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.水滴尺寸分布决定彩虹清晰度：均匀的0.5–2毫米球形水滴产生鲜明虹弧；雾气则导致宽泛、黯淡的光晕。
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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.主虹出现在反日点周围42°处，要求观察者、太阳与雨区在三维空间中精确共线。
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Secondary rainbows form from two internal reflections, appearing fainter and reversed in color sequence at 51° elevation.副虹由两次内反射形成，亮度较弱、颜色顺序相反，出现在51°仰角处。
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Urban light pollution suppresses rainbow visibility, making them rarer in megacities despite frequent showers.城市光污染削弱彩虹可见度，致使特大城市即便阵雨频发也难见彩虹。
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Aerosol loading from pollution or wildfires scatters light, reducing contrast and often eliminating the supernumerary fringes.污染或野火产生的气溶胶加剧光散射，降低对比度，常使附属虹纹消失。
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Polarization measurements confirm rainbow light is partially linearly polarized—evidence of its reflection-dominated origin.偏振测量证实彩虹光呈部分线性偏振，印证其以反射为主导的成因。
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Mountainous terrain enhances occurrence by triggering orographic lift and localized convective showers.山地地形通过地形抬升与局地对流性降雨，提升彩虹出现概率。
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Meteorologists now use rainbow geometry in lidar calibration to validate atmospheric particle sizing algorithms.气象学家现利用彩虹几何特性校准激光雷达，验证大气粒子尺寸反演算法。
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Its transient beauty masks a rigorous optical geometry—governed by Snell’s law and ray tracing mathematics.其转瞬即逝之美背后，是严格的光学几何——由斯涅尔定律与光线追踪数学所支配。
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Recognizing this phenomenon cultivates attention to light–matter interaction as a daily observable physics lesson.认识这一现象，有助于我们在日常生活中关注光与物质相互作用这一物理课。
精读 · 大约 10 分钟
听 → 读 → 学表达 → 懂 → 今天会说这一句 → 跟读录音 → 自己说一句。
③ 学 · 重点表达
- Mountainous terrain enhances occurrence by triggering orographic lift and localized convective showers. 山地地形通过地形抬升与局地对流性降雨，提升彩虹出现概率。
- Polarization measurements confirm rainbow light is partially linearly polarized—evidence of its reflection-dominated origin. 偏振测量证实彩虹光呈部分线性偏振，印证其以反射为主导的成因。
- Urban light pollution suppresses rainbow visibility, making them rarer in megacities despite frequent showers. 城市光污染削弱彩虹可见度，致使特大城市即便阵雨频发也难见彩虹。
④ 懂 · 选一选
⑤ 今天会说这一句
Mountainous terrain enhances occurrence by triggering orographic lift and localized convective showers.
⑥ 跟读并录音
Mountainous terrain enhances occurrence by triggering orographic lift and localized convective showers.
第一版：录下来、回听、再听标准音。系统打分以后再加。
⑦ 说 · 自己来一句
用今天的表达，自己说一句。
例子：Mountainous terrain enhances occurrence by triggering orographic lift and localized convective showers.
已记下。连续天数以后会显示在课表里。
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