Question: I live near a lake and have always enjoyed the different colors I see in the water. I get that the color is pretty much a reflection of the sky, but I’m curious whether pure water has any intrinsic color. Sure doesn’t look like it when a pour myself a cold glass, but maybe the color is just too faint? — BT, Woodruff, WI
Answer: Surprisingly, pure water does indeed have a color, but you won’t notice it in a glass. That’s because (as you suspected) the color is too faint. But if you look through a longer distance in water (around 10 ft or so) the blue color will be apparent. The graphic shows how light changes color as it travels through a long tube of water.
Molecules of H20 readily absorb the longer wavelengths of visible light (RED) and convert that energy into heat, warming the water. Shorter wavelengths (BLUE) move largely unimpeded through water. Whatever your eye sees looking through that column of water will thus be tinted blue.
Here’s a chart showing the light absorption properties of pure water. The horizontal axis show the color of light, and the vertical axis shows the coefficient of absorption (αW):
You are correct that the color of water in a lake or ocean is heavily influenced by the color of the sky. See my September 2020 post for a great slide show on the colors of lakes. But there are other factors involved, as you’ll see in the second slide show.
For essentially pure water, all you’ll see is the bluish tint. If you’ve ever seen an indoor swimming pool, and it’s sides and bottom were painted white, the water looks blue — even without a blue sky overhead. The light your eyes see has passed through that water twice — on its way down from the ceiling lights to the bottom of the pool, and again after it reflects upward toward your eyes. That’s enough distance for the blue tinting to kick in. Check out this photo of an indoor pool:
Note that the deeper water in the pool looks obviously blue, but the shallower water in the floating bucket looks only faintly blue. So that answers your question, BT, but let’s takes the science a little further …
The same effect can be seen in a pane of transparent window glass. Most windows are only 3–6 mm thick, and they’re 96% transparent to visible light. But if you take a loose piece of window glass and look through it edgewise, you’ll see a greenish tint too faint to be visible when looking through it normally. That color comes from iron oxide impurities in sand melted to make the glass.
And finally, air in a glass bottle looks fairly transparent, and has no intrinsic color. But in the atmosphere, air can appear in any color of the rainbow. Both absorption and scattering play a role here. See my post Why Most Sunsets are Red for more details.
Talking about the “color” of a substance normally considered to be transparent might seem contradictory, but the color of any substance depends of how it interacts with the white light spectrum available from our Sun. And those interactions can be weak or strong, depending on the molecular composition of the substance.
Next Week in Sky Lights ⇒ Why Snow Sometimes Squeaks