十万个为什么·科学启蒙30篇（4）
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Why Seawater Appears Blue
为什么海水看上去是蓝色
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Pure water absorbs red, orange, and yellow light more strongly than blue light, which scatters and reflects back.纯水对红、橙、黄光的吸收强于蓝光，而蓝光则易散射并反射回来。
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In shallow, clear seawater, this absorption-scattering effect makes the ocean look vividly blue under sunlight.在浅而清澈的海水中，这种吸收-散射效应使海洋在阳光下呈现鲜明的蓝色。
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Suspended particles like plankton or silt change the color—greenish near coasts, brownish after storms.浮游生物或泥沙等悬浮颗粒会改变海水颜色——近岸偏绿，风暴后偏褐。
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The sky’s reflection adds brightness but isn’t the main reason; even under gray clouds, deep water stays blue.天空倒影虽增添亮度，却非主因；即便阴天，深水依然呈蓝色。
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Very deep or murky water appears dark blue or black because almost all visible light gets absorbed before returning.极深或浑浊的水体呈深蓝或黑色，因为几乎所有可见光在返回前已被吸收。
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Scientists measure ‘optical depth’ to predict how far down blue light travels before fading completely.科学家通过测量‘光学深度’来预测蓝光完全衰减前能穿透多深。
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Coral reefs look turquoise because white sand reflects light upward while water absorbs longer wavelengths.珊瑚礁呈青绿色，是因为白色沙滩向上反射光线，而水体吸收了长波长光。
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This same principle explains why glaciers and thick ice appear blue—not from impurities, but from selective absorption.同一原理也解释了冰川和厚冰呈蓝色的原因——并非杂质所致，而是选择性吸收。