Quételet Colours on a small still pond  seen by Bjørn Revil near Oslo, Norway July 26, '08. Images ©Bj�rn Revil, shown with permission.

Still water covered with a thin layer of dust, pollen or algae often show rings, bands or regions of iridescent colour. Dusty windows show the same effects - don't clean your windows!

Quételet scattering, named after Adolphe Quételet more known for his statistics and sociology, is responsible.

The essentialas are small particles near a reflective surface. The colours come from interference between two sets of waves. (1) Waves from light first scattered by a particle and then reflected by the mirror before entering the eye. (2) Waves scattered by the particle from light that has first been reflected by the mirror.  Light waves along the two paths interfere constructively (brightness) or destructively (darkness) depending on the sun and observation angles and the light wavelength (colour). A complete Quételet display is a system of coloured fringes often accompanied by a corona produced by light scattered directly from the particles.