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We know that the single rainbow we see is actually a continuous cone of rainbow. If so, why don't we see that cone instead of a single, far-away rainbow?

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When sunlight enters a (roughly) spherical drop of water, it will be refracted at the entrance point, perhaps reflect one or more times off the interior surface of the drop, and then will be refracted again at the exit point. A significant proportion of the light enters, reflects once internally, and then exits, leaving at an angle of about 41° from the incoming ray. For a single drop, the light that follows such a path will emerge in a cone, centered on the sun and with a half angle of 41°.

When the light refracts at the air/water interface, the angle is wavelength-dependent. The result is that the cone is actually a conical rainbow, ranging from an inside blue cone at an angle of 40° to an outside red cone at an angle of 42°.

Of course, you can't see the rainbow from a single water drop, because you only see light that makes it to your eyes. So, the color (if any) you see from any one drop is the part of that drop's rainbow cone whose light hits your eye. The result is that your eye sees colors coming from a reversed cone of drops; drops at an angle of 42° between you and the sun shine red, those at an angle of 40° shine blue, and those between shine intermediate colors.

Now, why can't you see the cones of color as actual cones? Because the cones' apexes are all at your eye, so there's no way for your eye to see the depth of the cone.

And why does the light always bounce exactly once internally? Answer: it doesn't. Some light bounces twice and comes out at a slightly different angle, forming a double rainbow. Other numbers of bounces form other rainbows, but they're generally difficult to see.

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  • $\begingroup$ Note also that you can't use optical parallax to judge the "distance to the rainbow" because (as you correctly point out) all the light rays are entering your left eye and your right eye at the same angle. Since there's no discernible parallax between the two views, the rainbow appears to be "very far away." $\endgroup$ – Michael Seifert Oct 31 '18 at 13:27
  • $\begingroup$ @ Michael: The parallax of the rainbow is discernibly zero. Zero parallax means the observed image is infinitely far away. For example, in geometrical optics, a zero parallax image is formed at infinity behind a lens, if the object distance is equal to the focal length. $\endgroup$ – jkien Nov 10 '18 at 22:54
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The difficulty is telling them apart.

By construction, these "several rainbows" overlap in the field of view of the observer and, in general, especially for atmospheric rainbows, distances are usually large so it's hard to tell from where exactly the rays you see are emerging, even if the necessary droplets and lighting happen to be present in a large range.

However, you can create a rainbow with a garden hose, and then the rainbow is close to you and it's easier to test for the existence of the cone.

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An interesting option is to treat your question as a simple geometrical optics problem with a virtual image at infinity. Then, the rainbow image produced by the rain curtain is the single apparent origin of the light rays received by observers, either eye or camera, at different observation locations. That apparent origin is a single 42° circle at infinite distance (because parallel rays appear to originate from a single "point" at infinity).

So your question why the eye does not see a cone instead of a single, far-away rainbow, is answered by pure geometrical optics. It is similar to looking in the mirror and asking why the eye sees an image, instead of the light rays and the particles of the mirror.

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