In the late afternoon of December 15, 2007, temperatures began to stay below freezing 24 hours a day for the first time (!) in this year. Already the next morning, many of the small puddles on a lane near our town Barsinghausen were covered with ice. Children enjoyed breaking these crusts of ice with their feet. But at some of the puddles which had frozen down to the ground, they had no success and just caused some fissures in the ice. These fissures showed extremely intense colours.
The colours are probably caused by interference. Light enters the ice and is reflected to and fro between the edges of the fissures. So there are differences in the time the light needs to cover the distance between the ice and the eye of the observer. This causes light waves to amplify or extinguish each other. This is what we perceive as colours.
There is no direct sunlight necessary for this effect (1 2), but it can increase it enormeously (3 4).
Author: Reinhard Nitze, Barsinghausen, Germany