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Why the colors of Secondary rainbow is reverse of that in the color in the Primary rainbow?

What can be the possible reason among the following options

  • Because it is formed by one internal reflection.
  • Because it is formed by two internal reflections.
  • Because it is formed by refraction and one internal reflection.
  • Because it is formed by refraction, dispersion and one internal reflection.
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The Wikipedia article on Rainbows offers a number of supporting images, but to my surprise does not offer either an explanation or an image to support one. –  dmckee Dec 8 '12 at 18:50

2 Answers 2

up vote 4 down vote accepted

When a primary rainbow is formed, it is due to one total reflection in water droplets. A secondary rainbow is formed by light that underwent two internal reflections, and that is what changes the ordering of the colours. You can also have a tertiary rainbow formed by three internal reflections that would have the same ordering as primary and so on.

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Actually, total internal reflection is impossible in spherical raindrops; an internal reflection occurs, but it is not total. And the rainbow is not just an arc, it is a full disk. The disks are sized differently for each color, and are brightest at the very edge, which is what makes the colored arcs appear.

For the primary rainbow, these disks are centered at the anti-solar point, which is the point you'd see looking directly away from the sun. The violet disk is 40 degrees wide, and the red disk is 42 degrees, so the red arc appears outside of the violet. For the secondary, the disks are centered on the sun - away from the direction you are looking when you see it. The violet disk is 127 degrees wide, and the red disc is 130 degrees wide. Because they are bigger than 90 degrees, these disks wrap around the sky so that you see the secondary's colored arcs in the same direction you see the primary's arcs. Technically, the red arc is still outside of the violet arc, but you see them reversed because of this wrapping.

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A diagram would go a long way here. –  Brandon Enright Mar 4 at 16:03
"total internal reflection is impossible in spherical raindrops; an internal reflection occurs, but it is not total." I suppose that I understand what you mean by that, but if so it needs some qualification. If you mean it in an absolute sense then I believe that you are wrong. –  dmckee Jun 28 at 17:22

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