HELEN CZERSKI ON … RAINBOWS IN ICE
“HE FISHED OUT AN AXE, HACKED INTO A GLACIAL PIMPLE, AND OUT CAME ICE FILLED WITH RAINBOWS”
We love it when a gift that we’ve carefully chosen makes someone happy. But when the recipient of the present is a small child we have to accept that the real source of joy is often the packaging rather than its contents. Physicists have a similar streak, and can often be left in a corner happily playing with something that isn’t meant to be the main attraction. This time, the culprit was one particular chunk of ice, on a day that had been filled with the stuff.
The star of the show was a cave in the side of a glacier, and we had spent an afternoon working there, filming for a BBC documentary. But right at the end of a long day, when everyone was tired and ready to go home, our guide said a child had once told him there were rainbows in the ice at one specific spot next to the cave. He fished out an ice axe, hacked into an unassuming glacial pimple, and out came a chunk of ice filled with rainbows. Except that wasn’t quite it – there were sheets of blues, pinks and greens, glinting inside the clear ice. And once I had been given this toy to play with, I didn’t want to leave.
As the colours in the ice were the same as those seen on soap bubbles, they were a dead giveaway of what was going on. Light hitting a thin soap film can reflect off both the near and far surfaces, and when those two reflections overlap with each other, some colours of the rainbow are enhanced and some suppressed. In any one spot, we just see one colour, the one that comes from the specific mix of the rainbow that’s on offer from that place. And only some combinations are possible, which is why blue, pink and green dominate. But there was no soap in the ice to cause the effect. Instead, there must have been incredibly thin, sheet-like cracks, and light was reflecting off the near and far surfaces of the cracks. The blue colours told me that some of these cracks were only a few hundred nanometres thick, and as they widened, the colours merged into yellow, then pink, and then green. The colours let me see the width directly. But ice cracks all the time. When you put an ice cube into a drink, you quite often hear the popping sound as cracks form. In this case, it’s because the outer ice expands slightly as it warms (before it melts) and the mismatch in size between inside and outside forces the structure to break. So why do we never see colours in an ice cube? I suspect that it’s because the cracks in an ice cube are too wide – they just open up until the inside and outside of the ice cube are the same size. My guess is that the cracks in this glacial ice followed hidden layers of stress, possibly left over from the ice flowing sideways under pressure. Instead of pulling apart, the two surfaces were just shunted over each other, leaving a truly tiny gap in between. The final sideways shimmy might even have been caused by the impact of the guide’s ice axe.
As I held the ice, I could see the colours fading as the warmth of the sunlight softened the ice and the cracks vanished. The guide commented that this was the most excited he had seen me all day. I could have spent hours there, looking at the prettiest fracture patterns I had ever seen. But everyone else was hungry and waiting for me, so eventually I had to leave. Still, I carried my excitement home with me, because the unexpected colours in the ice had been the best toy of the day.