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A dewbow is an optical effect, similar to a rainbow where the dewdrops reflect and disperse sunlight, producing all the colours in the visible spectrum.
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Occurrence
Although rare, dewbows are most common in autumn, when dewdrops form on a dense network of spiderwebs (or rarely on walls, grasses and other vegetation), where the dew hanging on the webs acts like rain, and diffracts the light.
Shape
A dewbow is centred on the antisolar point but as the dew is on a horizontal plane, the bow is not a circular arc, but often elliptical or an open arc shape in a parabola or hyperbola. The precise shape of the bow depends on the elevation of the sunlight, and any slope present on the dew covered ground.
References
Dewbow Wikipedia(Text) CC BY-SA