A rainbow may be seen when there are small spherical water drops in the air and the sun is low in the sky behind you. Sunlight entering the surface of a water drop is refracted, then reflects off the rear of the drop, then is refracted again as it leaves the drop on its way back to your eye. White sunlight is a combination of all the colors of the spectrum, and each color is refracted or bent at a slightly different angle. Thus each color is seen at a different place in the sky resulting in a rainbow. In a similar fashion the mist from a waterfall can form a spray bow.
Urriðafoss has the greatest flow of any waterfall in Iceland, about 360 m³/s (13,000 cu ft/s) over a 6 m (20 ft) drop. There are plans to build a dam and hydropower plant just upstream, whereupon this waterfall will disappear.
Urriðafoss, Þjórsá, Ísland (Salmon Falls, River Thjorsa, Iceland) Locate on Google Maps
Nikon D100, Nikkor AF-S 24-85mm at 42mm, f/13, 1/100 sec