Frost Kaleidoscope ~ Another polarised light image (see pond ice) by Dieter Zawischa (website 'Origins of Colour'). ©Dieter Zawischa, shown with permission.
Dieter's image is of window frost crystals produced artificially. "Due to double-glazed windows and modern heating systems, I have not seen beautiful frosted windows for decades. Therefore, when I visited an exposition at the "Universum Science Centre" in Bremen this year, I was glad to see artificial fern frost on a cooled glass plate between polarisation filters, and I took some pictures."
Fern (or window) frost achieves its beauty from the large flat single crystals of ice that grow across the glass and meet in gentle curves. See earlier OPODs (1,2) for some images.
But - Ice is birefringent or double refracting as demonstrated in Dieter's earlier OPOD image.
Here, placing the frosty glass between two polarizer sheets has revealed the colours that result from interference between the two birefringent rays.
Fern frost still occurs even in these days of double glazing. Look on a car windscreen or bodywork for example. Use a polarising filter and experiment by viewing the frost at different angles both through the windscreen and by light reflected from it. There are likely to be positions where the colours are revealed. Once you see them, rotate the polariser - if birefringence is the culprit then the hues will change.