STEM与日常科技·英语30篇（5）

13 / 30

为什么有些彩虹是双层的，而有些不是？

A double rainbow forms when sunlight reflects twice inside raindrops instead of just once.
The second reflection makes the outer arc dimmer and reverses its color order compared to the primary bow.
Because light loses energy with each internal reflection, the secondary rainbow is always fainter than the first.
You can only see a double rainbow when the sun is low and rain fills the sky opposite it.
The dark band between the two arcs, called Alexander’s Dark Band, occurs because no light scatters there.
This phenomenon follows precise optical laws involving refraction, reflection, and dispersion in spherical water droplets.
Unlike artificial rainbows from mist or sprinklers, natural double bows require uniform droplet size and angle alignment.
Photographers often wait for specific cloud gaps to capture both arcs clearly without glare.
Scientists use this effect to calibrate atmospheric sensors that measure droplet distribution in clouds.
Even though rare, double rainbows prove how consistently physics shapes everyday sky phenomena.