Shadowed Bow
Imaged by Tim White near Newport, Wales.  
©Tim White, shown with permission.

This bow shows beautiful colours.  One could believe that they are pure spectral colours but in fact all but the extreme reds are colour mixtures.

The dark shadow is a form of the ‘rainbow wheel’ effect.    The rainbow is formed by raindrops in the half of the sky opposite the sun glinting light towards the eye or camera.    To glint they must be in sunlight and not in the shadow of a cloud.

 Although a cloud shadow is a roughly parallel sided tube of dark unlit air it appears to converge with distance to a point directly opposite the sun, the anti-solar point.    Sometime the shadow and adjacent bright sunlit air is visible as anticrepuscular rays.

When the shadow falls over raindrops forming a rainbow the effect is more extreme.   The drops no longer glint and the rainbow appears cut off.   Rainbow light extends well inside the coloured rim as a white glow and this is cut off too leaving a dark wedge in the sky.    Several small cloud shadows can give the effect of a spoked rainbow.

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Rainbow wheel or spokes

Several small clouds can cast shadows over the rainbow forming raindrops to form a spoked effect.

The clouds can be in the opposite part of the sky rather than close to the bow as shown here.