KnowHOW team explains: It’s a common misconception that the blue colour of pure ice (in glaciers or polar premafrost) is due to the same phenomenon that makes the sky blue ? light scattering. In frozen water and in the sky the processes are almost the reverse of each other.

A blue sky results when light bounces off molecules and small dust particles in the atmosphere. Because blue light scatters more than red does, the sky looks blue except in the direction of the sun (particularly when the sun is near the horizon and the blue light is scattered out of the sunlight, leaving the red colour of sunrises and sunsets).

When light passes through ice the red light is absorbed while the blue is transmitted. Were the operating process scattering as in the atmosphere, the transmitted light would be red, not blue. However, because of the large size of snow grains and ice crystals, all wavelengths of visible light are scattered equally. So scattering does not play a significant role role in determining the colour of the transmitted light.

It takes an appreciable thickness of pure ice to absorb enough red light, so that only the blue is transmitted. Actually, even in ice, molecules of water are constantly vibrating. They mop up longer, redder wavelengths of light far more effectively than blue wavelengths ? thus boosting the relative amount of blue in daylight scattered from it.

The question was sent by Bithi Sinha Roy from Asansol