Glacier ice and frozen lakes appear blue, yet icicles and ice from your freezer appear clear. Why is ice blue? The quick answer is that it's because water absorbs other colors of the spectrum, so the one that's reflected back to your eyes is blue. To understand why, you need to understand how light interacts with water and ice.
Key Takeaways: Why Ice Is Blue
- Ice appears blue because water is intrinsically turquoise blue.
- The color of ice deepens with increasing thickness and purity.
- Ice that appears white often contains a lot of air bubbles, cracks, or suspended solids.
Why Water and Ice Are Blue
In both its liquid and solid form, water (H2O) molecules absorb red and yellow light, so the reflected light is blue. The oxygen-hydrogen bond (O-H bond) stretch in response to incoming energy from light, absorbing energy in the red part of the spectrum. Absorbed energy causes water molecules to vibrate, which can lead water to absorb orange, yellow, and green light. Short wavelength blue light and violet light remain. Glacier ice appears more turquoise than blue because hydrogen bonding within ice shifts the absorption spectrum of ice to a lower energy, making it more green than liquid water.
Snow and ice that contains bubbles or lots of fractures appears white because the grains and facets scatter light back toward the viewer rather than allow it to penetrate the water.
While clear ice cubes or icicles may be free of the gases that scatter light, they appear colorless rather than blue. Why? It's because the color is too pale a blue for you to register the color. Think of like the color of tea. Tea in a cup is darkly colored, but if you splash a small amount onto the counter, the liquid is pale. It take a lot of water to produce a noticeable color. The more dense the water molecules or the longer the path through them, the more red photons are absorbed, leaving light that is mostly blue.
Glacial Blue Ice
Glacial ice starts out as white snow. As more snow falls, the layers below it become compressed, forming a glacier. Pressure squeezes out the air bubbles and imperfections, forming large ice crystals that permit light transmission. The top layer of a glacier may appear white either from snowfall or from fractures and weathering of the ice. The glacier face may appear white where it's weathered or where light reflects off the surface.
A Misconception About Why Ice Is Blue
Some people think ice is blue for the same reason as the sky is blue - Rayleigh scattering. Rayleigh scattering occurs when light is scattered by particles smaller than the wavelength of the radiation. Water and ice are blue because water molecules selectively absorb the red part of the visible spectrum, not because the molecules scatter the other wavelengths. In effect, ice appears blue because it is blue.
See Blue Ice For Yourself
While you may not get a chance to observe a glacier firsthand, one way to make blue ice is to repeatedly poke a stick down into snow to compress the flakes. If you have enough snow, you can build an igloo. When you sit inside, you'll see the blue color. You can also see blue ice if you cut a block of ice from a clean frozen lake or pond.
- Braun, Charles L.; Sergei N. Smirnov (1993). "Why is water blue?". J. Chem. Educ. 70 (8): 612. doi:10.1021/ed070p612