STEM与日常科技·英语30篇（5）
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Why Some Rainbows Appear Double While Others Don’t
为什么有些彩虹是双层的，而有些不是？
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A double rainbow forms when sunlight reflects twice inside raindrops instead of just once.
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The second reflection makes the outer arc dimmer and reverses its color order compared to the primary bow.
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Because light loses energy with each internal reflection, the secondary rainbow is always fainter than the first.
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You can only see a double rainbow when the sun is low and rain fills the sky opposite it.
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The dark band between the two arcs, called Alexander’s Dark Band, occurs because no light scatters there.
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This phenomenon follows precise optical laws involving refraction, reflection, and dispersion in spherical water droplets.
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Unlike artificial rainbows from mist or sprinklers, natural double bows require uniform droplet size and angle alignment.
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Photographers often wait for specific cloud gaps to capture both arcs clearly without glare.
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Scientists use this effect to calibrate atmospheric sensors that measure droplet distribution in clouds.
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Even though rare, double rainbows prove how consistently physics shapes everyday sky phenomena.