HE­LEN CZ­ER­SKI ON … RAIN­BOWS IN ICE

BBC Earth (Asia) - - Comment & Analysis -

“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.

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