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.
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In shallow, clear seawater, this absorption-scattering effect makes the ocean look vividly blue under sunlight.
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Suspended particles like plankton or silt change the color—greenish near coasts, brownish after storms.
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The sky’s reflection adds brightness but isn’t the main reason; even under gray clouds, deep water stays blue.
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Very deep or murky water appears dark blue or black because almost all visible light gets absorbed before returning.
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Scientists measure ‘optical depth’ to predict how far down blue light travels before fading completely.
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Coral reefs look turquoise because white sand reflects light upward while water absorbs longer wavelengths.
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This same principle explains why glaciers and thick ice appear blue—not from impurities, but from selective absorption.