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   Rainbow Cone 

The Rainbow Cone

Rays of the primary bow form a cone. Its tip is at your eye. Its axis is parallel to the sun's rays and directed downwards to the antisolar point.
Myriad raindrops near the cone's surface send sunlight into your eye to produce the bow. The drops can be a few feet away or a mile or so. Their distance does not matter, the rainbow looks the same. The rainbow is a collection of rays with particular directions, it does not otherwise exist and it is not located at any particular point in space.
Drops inside the cone brighten the sky inside the bow. Drops outside the cone send no light into your primary bow*. But they might be on the surface of someone else's rainbow cone. Each person has their own cone and sees their very own rainbow**.
*   The secondary bow also has a cone. Its half angle is 51º compared to the primary's 42º. Both cones share the same axis. Droplets on the secondary cone's surface or outside it contribute light to the secondary. Drops outside the primary cone and inside the secondary can send no light to your eye. The resulting dark sky is called "Alexander's dark band".
**   This becomes evident when a rainbow is seen from a car or train, it stays fixed relative to the sun while the landscape appears to move. Strictly speaking each eye sees a different bow but this is only apparent when the water drops forming them are close, from a garden hose for example.
***  The bow over Schlägl, Austria was imaged by Karl Kaiser, large version here.