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While camping in the desert north of Coober Pedy, South Australia, on 8 July 2007 my son and I were privileged to witness a white rainbow at daybreak (see photo, right). The landscape was covered in mist and the white rainbow’s arc seemed to grow as the sun came up, though it faded away as the mist evaporated. I’ve asked old swagmen, indigenous locals and several lecturers at two universities, but no one has ever witnessed the phenomenon. What caused it?

• This was a fogbow, sometimes known as a cloudbow or mistbow. Like a primary rainbow, it is centred on the point opposite the sun and has an angular radius of approximately 42 degrees. It is caused by the same mechanism: reflection and refraction of sunlight by water droplets. In this case the droplets are unusually small – less than 50 micrometres across – allowing diffraction to spread the bands of colours so that they overlap and appear white.

Occasionally, fogbows will show a bluish tinge to the inner edge and a reddish one on the outer. From some viewpoints, such as an aircraft, a fogbow can appear as an almost complete circle.

Storm Dunlop, Chichester, West Sussex, UK

• A fogbow is a frustrated rainbow, formed in essentially the same way. Sunlight destined to create rainbows and fogbows is refracted twice – once as it enters a water drop and again as it leaves. While inside the water drop between the two refractions, the light bounces off the inside back surface, sending it heading back towards the sun. This is why rainbows and fogbows are seen when the sun is behind the observer. The path of blue light is bent more than red by the droplet causing sunlight to be dispersed into the colours of the visible spectrum with blue at the bottom of a rainbow and red at the top.

Rainbows appear white when water droplets are less than 100 micrometres across – small enough for diffraction to dominate over refraction. Each water drop forms its own diffraction pattern – bands of alternate constructive and destructive interference – for each colour: the smaller the drop, the broader the bands. When the drops are small enough, these bands become so broad that all the colours overlap, essentially mixing them all together again, to make white.

Beautiful images can be found at .

Mike Follows, Willenhall, West Midlands, UK