When Water Is Green

This weekend is St. Patrick's Day, making it an appropriate time to have a look at why water sometimes appears as green. Niagara Falls is where green water is often to be seen. I recall one childhood day, I was on the Canadian side of Niagara Falls some distance upstream from the falls. The sun was high in the sky, and the water of the river was a vivid green. It was almost like the margins of this blog.

Ever since then I have had this idea of green water in my mind, and now I have finally gotten around to giving it some thought. Underwater plants are not the reason for water appearing green, because the water is light green and underwater plants tend to be particularly dark.

The reason that deep water tends to appear as blue is the fact that water absorbs light, but not all colors (colours) equally. The spectrum, from longest wavelength to shortest, is: red, orange, yellow, green and, blue.

Water is transparent because it's molecules line up due to hydrogen bonding so that light can pass between the molecules. Hydrogen bonding takes place because one side of the water molecule is more positive, and the other side more negatively-charged. This causes water molecules to line up positive-to-negative.

Blue light is the shortest in wavelength, meaning that it's wavelength has the easiest time "fitting through" the gaps between the lined-up molecules. Red, with the longest wavelength, has the most difficult time. Thus, red light is absorbed first by the water.

You may have noticed that, in underwater photography, there is nothing red below about 9 meters (30 feet). This is because red light has been absorbed by the time light from above reaches this depth. Deep water appears as blue because only blue light can pass through enough water to be refracted back to the surface.

Green light is next, after blue, for the shortest wavelength. Green light does not get through enough water to be refracted back to the surface by deep water. But green light can be reflected back to the surface if, in relatively shallow water, there is something to give green light a "boost' by reflecting it back. This then gives green light an "advantage" over blue, so that the water appears green.

Relatively shallow water, with a mostly-bare rock bottom, will tend to appear green. The water must be deep enough so that the bottom cannot actually be seen, and that a significant amount of refraction does take place. Green light, with it's longer wavelength than blue, gets through the water-air interface easier than blue so that green water typically appears more vividly green than blue water appears blue. Water upstream from the falls, around the Grand Island Bridges, tends to appear as more of a dull green, but this is because there is less bare rock on the river bottom.

This water-air interface, manifesting what we refer to as surface tension in the top layer of water molecules, is partially reflecting and reflects a lot of light back down. Some fish in shallow water can see what is around them by looking at this reflection from the surface. This operates in a way similar to a store window, in which you can see your reflection, it is partially transparent and partially reflecting. But electromagnetic waves, including light, are reflected by objects that are similar in size to their wavelengths and this gives green light, with it's longer wavelength, the advantage in getting through the water-air interface.

Turbulent water, although not white water, also tends to appear as green. This is also due to reflection from below. Tiny air bubbles in the water act as collective mirrors that reflect green light back to the surface.

Look at the photo at the top of this local newspaper. Water going over the falls, as well as the water about to go over, is a definite green http://www.niagara-gazette.com/ . Water in the gorge, downstream from the falls, also appears as green. The water in the gorge is very deep, so the green must come from reflection from underwater air bubbles rather than from the river bottom.

Water can rarely appear as blue-green if there is both shallow and deep water in close proximity. The deep water refracts blue back to the surface, while the nearby water reflects green back up. If you look at the article about Niagara Falls on http://www.wikipedia.org/ , or do a web search for "Niagara Falls Photos", it is easy to see that in the gorge below the falls, the more turbulent the water is, the more likely it is to appear green rather than blue.