Walking On Water: Physics of Clear Ice

A video went viral yesterday showing two hikers in Slovakia walking across a frozen lake. What is amazing about this lake is the fact that it froze crystal clear such that the hikers appear to be walking on calm water.

In cased you missed it, here is the video:

Now, like any self-respecting scientist, I tried to get APS approval to repeat their hike, you know, just to verify the experimental results, but sadly no can do. The stark and beautiful scenery of the Slovakian mountains will have to wait until another day.

While I was daydreaming, I wondered what caused this particular lake to freeze clear? What’s special about its conditions? And can I make clear ice at home? As usual, physics has the answers.

A substance looks clear or transparent when visible light can pass through it without being absorbed, reflected, or scattered too much.

Water does absorb some wavelengths of light, but crucially not visible light, as this plot of absorption across wavelength makes clear (ha). If we could see in ultraviolet wavelengths, then water would not appear clear because most of the light would be absorbed before reaching our eyes. The features in this graph are caused by the internal electron structure of the water molecule, which determines what energies of light can excite water.

Absorption spectrum of liquid water. Credit: Kebes via Wikimedia Commons

This explains why liquid water is clear, but then why is the ice in your freezer and the ice on most frozen lakes cloudy?

Cloudiness, or opaqueness, can also be caused by light scattering. Common forms of ice have lots of mineral impurities and trapped air bubbles frozen in amongst relatively small ice crystals. These impurities and small crystals create many surfaces within the ice that can scatter light. Since light cannot travel in a straight line through the ice, it appears cloudy.

But with the exact right conditions, it is possible to form clear ice. The key is to freeze water very slowly so that impurities and air bubble have a chance to rise to the surface and, in the case of air, escape. A slow freezing also allows bigger ice crystals to form, which have few surfaces to scatter light.

Veľké Hincovo pleso in the High Tatras, Slovakia. Credit: Michal Klajban via Wikimedia Commons

In the case of this natural glacier lake, it is likely that the water is very pure. On a map, Veľké Hincovo pleso doesn’t appear to have any rivers feeding into it, so clean rain is probably its main source of water. A lack of impurities and gases is crucial to forming clear ice.

This lake may have then frozen slowly at high temperature close to 32 degrees Fahrenheit, perhaps even in layers as it rained and then froze.

Finally, the hikers in the video appear to walk on a fairly shallow portion of the lake. This thin layer of ice means light has less distance to travel and thus less chance to scatter before reaching your eye.

Partly because it is so temperamental to form, clear ice is captivating and highly prized by ice sculptors and bartenders in swanky restaurants. Professional ice machines often freeze agitated or flowing water slowly in layers to create that crystal clear ice.

Clear ice sculpture. Credit: Cropped from Tom & Katrien via Flickr

If you’re interested in creating clear ice at home, then check out this list of clear ice experiments created by Camper English for his cocktail science blog Alcademics. He has lots of low-tech recipes that you can use to impress your friends this Christmas.

And if you find ice fascinating, you wouldn’t be alone. Even today scientists are still studying how ice crystals form. Researchers have just managed to create a new form of ice known as ice XVI, as published yesterday in Nature.

By Tamela Maciel, also known as “pendulum”

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