Breaking the Ice

Dieter and I have been having a friendly discussion on the origin of colours in pond ice. Are they from the birefringence of the ice itself or from interference of light across thin fissures? Or both?

Science is about experiment rather than opinion and so Dieter attacked a nearby icy pond. �Instead of a polarising filter, yesterday I took a hammer with me when I looked for the ice on a nearby pond. What I did was not very inventive - knowing Eva Seidenfaden's site I just wanted to reproduce her results (1,2,3) on induced fissures in ice.

The ice was not very thick and not really smooth, but nevertheless I succeeded in producing cracks which did not get filled with water immediately and I could see the interference colours and even get some photographs. [Importantly] no polarising filter was used!

On one of the pictures [the top one] you see the impression of my hammer in the upper right corner. It was dull, grey weather, and I applied only very little post-processing.

This time, there is no discussion on the way the colours arise!�

The colours are a result of thin film interference as is seen in soap bubbles and oil films on a puddle of water except here the 'film' is a thin air filled fissure in ice.

Dependent on the local fissure thickness and the exact angle of view there is a phase difference in the arrival of light waves at the eye from reflections at the upper and lower fissure surfaces. When the two waves are in phase they reinforce and we see light. When out of phase the wave amplitudes cancel and there is darkness. The phase condition is also wavelength dependent - hence the colours.

Ice colours in pond ice can arise from interference in narrow fissures, from birefringence and under the right lighting conditions from refraction by trapped bubbles. A polarising filter can help distinguish between the first two sources as birefringence shows strong colour shifts when the filter is rotated.