Science Puzzle
Why Is Glacier Ice Blue?
Snow is white. An ice cube in a drink is clear. Yet the ice deep inside a glacier, seen in a crack or on the face of a freshly broken iceberg, glows a vivid blue. All three are frozen water.
Why is glacier ice blue?
The Answer
Ice soaks up a little red light. Through a few centimetres you'd never notice, but light that travels through metres of glacier ice loses most of its red, and what's left looks blue.
White light is a mix of every colour. As it passes through ice, the red end of the spectrum is absorbed: in a couple of metres of ice, red light is cut by more than half, while blue light can travel hundreds of metres. An ice cube is far too thin for this to show, so it looks clear. Glacier ice is many metres thick, and light that has wandered through it comes out blue.
Snow is made of the same ice, so why is it white? Snow is full of air. Every boundary between ice and air bounces light back, and fresh snow has countless boundaries, so light is sent back out before it gets deep enough to lose its red. All colours come back together, and that looks white. Deep in a glacier, the weight of the snow above squeezes out the air bubbles, so light can travel a long way into the ice.
That's also why the bluest ice is found where a glacier has just cracked or calved, showing dense, bubble-free ice from deep inside.
The principle: Absorption of red light. Water and ice absorb a little red light, so light that travels through metres of clear, bubble-free ice loses its red and looks blue.