Colours in a frozen lake seen by Andrew Kirk. Image ©Andrew Kirk, shown with permission.
Now that winter is closing fast on the Northern Hemisphere look for optical effects on icy ponds – don’t fall in!

The large oval structure is an air bubble, about one meter long. The bubble appears to be trapped between two layers of ice; the smaller bubbles (with their specular highlights) are casting shadows onto the lower ice layer. The iridescence was visible to the naked eye but really popped with a circular polarizer on the camera.

The iridescent colours are produced by a structural colour effect, thin film interference, the same one that gives us the metallic hues of soap bubbles and of oil films on puddles.

Here the necessary thin film could be air narrowly trapped between two ice layers or an ice fissure. Light waves can be reflected upwards within the upper ice layer. Others leave the ice, traverse the thin air layer, reflect off the lower air/ice surface and enter the upper ice again.

If the air layer is thin enough the two wave paths combine. When the wave crests coincide there is light, when they are completely out of phase there is darkness. The light or darkness condition depends on the film thickness and, importantly, on wavelength. Thus, some colours are enhanced while others are darkened. The final ones we see are always complex mixtures – hence their non-spectral metallic hues.

Another possibility is that the thin film is water trapped in a fissure or between two ice layers. Any thin film of different refractive index to the material bounding it can produce interference colours. More ice colours come from weak birefringence, from classical refraction/reflection by air bubbles trapped within the icy mass and because ice is slightly blue by absorption ~ but you will need a crevasse or ice cave to see that.


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