Drops on the edge of a cone of half angle 42° form a rainbow. The cone axis extends from the eye towards the antisolar point.

Rainbow forming drops can be any distance along the cone. The rainbow is always effectively at infinity.

A full 360° bow is seen when there are enough drops along every part of the cone surface. This usually means that the ground is not in the way
Dettifoss Waterfall, Iceland - Bill Belko imaged the rainbows conjured by Europe's mightiest waterfall. The glacial river Jökulsá á Fjöllum generates 44m high Dettifoss (Niagara 53m) in northern Iceland's Jökulsárgljúfur National Park. Sediment from the Vatnajökull glacier renders the water turbid.  Images ©Bill Belko, shown with permission.
Bill's shadow is at the bow's centre. He captured the panorama with a Canon EOS 20D with a Sigma circular fish eye 180deg 8mm 1:4EX. The 84° diameter rainbow is always circular and the slight distortion in the image arises from the necessarily wide lens.
Lower image: Shows Alexander's Dark Band between the primary and secondary bows. Water drops cannot deflect light into this region except for a miniscule amount reflected directly from their outer surface.

The primary bow colours are slightly desaturated. Spray and mist droplets from the falls are smaller than raindrops. Bows get broader and more pastel coloured as the drop size decreases. Diffraction makes them increasingly diffuse until eventually we see a fog or cloudbow.
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