What is the Color of Water?

The Question

The answer to the question I just poised has, I know, extreme political and cultural ramifications.  It is so sensitive and controversial of a subject, that I take great personal and professional risk by broaching it here.  I'm edgy.  But it's too late now.  If you really want to see how deep the rabbit hole goes, take the red pill and let's go.

Author's photograph

Wrong Answers

1. Water is blue because the sky is blue.
2. Water is colorless.  Just look at a glass of it.
3. The blue of oceans and lakes is an illusion; they aren't colored at all.
4. Water must be some bizarre sort of white or grey.  Look at a cloud.

False, false, false, false.

You may cherish one of the misconceptions above (probably numbers 1 or 2, and even 4 might seem intuitive).  But I am afraid you are mistaken.  What is left?

Let's consider each of these to their proper necessity, and we'll be left with the true answer.

Why the Answers Above are Wrong

Number 1 is a stock answer, and it is the most fantastically wrong.  It does, however, have the simple appeal that the sky color "explains" the color of bodies of water.  To disprove this, you need only to go out to a lake on a cloudy day.  Or at sunset, when the sky is no longer blue but yellow, orange, and pink.  You can also go out at night and shine a powerful light into the water.  You can even look at the oceans from outer space, and in spite of the fact that they are seen through the sky, they are still blue (if blue is reflected back toward the water by the sky, how does it then slip out toward space through the same atmosphere).  In all cases, we find that the water is still blue.

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So Number 1 is right out.

Number 2 is consistent with our common close-up experience of water, and if you were a curious enough child, it might have even caused you some cognitive dissonance to contrast the obvious colorlessness of a glass of water with the striking blue of Lake Michigan or Lake Huron.  I propose you try a demonstration.  Fill a tall white bucket with clean tap water, and stay inside the house.  Go into the basement if you really want.  Flip on the lights (fluorescent work best), and stare down into the bucket.  Are you surprised?

I accidentally found this out last summer at work when I filled a tall, white bucket with water and looked down at it.  I showed it to my coworker Emily, who first said, "You put dye in it, didn't you?"

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So we can show by demonstration that Number 2 turns out to be untrue.

Number 3 is not worth talking about.  It's false.

Number 4 is maybe a reasonable answer, until you show how an atomized version of just about anything can make a whitish or grey cloud.  Take a can of black or blue spray paint outside on a sunny day and spray it (just for a second) straight up.  You'll notice that it looks light grey, with some color influence of the paint itself.  A dense cloud of any particle tends to have a high reflectance.  Water's visible spectrum reflectance as a thick mist runs from 70% to 95%, giving them their characteristic white hue.  Just think of the last really thick fog you had to drive through.  But when the fog droplets strike your windshield, they are colorless, like a glass of water.  (While on the subject, note that a cloud is a mist of water droplets, not water vapor.  Water vapor is truly transparent and colorless because it is a gas.  Clouds are made of liquid.)

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So Number 4 is out.

So what the hell color is water?

The answer may shock you.  You may be surprised.  You might even be a little pissed off.

Water is blue.

Now calm down, there's no need for that kind of language.  Our expectations are always being challenged, and sometimes it feels uncomfortable.  But our demonstration to disprove answer Number 2 was unequivocal.  Look into that bucket and you'll see a faint aquamarine.  Look into an indoor pool, even if its walls are painted white, and the effect stands out even more.  These observations speak for themselves: No matter what our close-up and personal experience with water, we must own up to the observation that it is, quite simply and uncompromisingly, blue.

So why does it appear clear in a glass, but blue on a large scale?  For the same reason that a single drop of green tea appears colorless, but a whole cup is a dark yellowish-green.  It's simply a matter of proportions.

Water, when struck with light, tends to absorb at below-visible and above-visible wavelengths.  It absorbs the least in the visible spectrum, which is why most visible light passes through unchanged.  It's also part of the reason why it takes so much heat to boil water; it absorbs tons of heat before a phase change.  Now water does not absorb equally.  It absorbs more than 10 times more red, orange, and yellow than it does green, blue, indigo, and violet.  So when light passes through water, the frequencies on the red side are preferentially absorbed, and the frequencies on the blue side are absorbed the least.  Look at the graph below.

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The intrinsic molecular mechanics of this are really complex, but we will simply say that water reflects light at a vibrational level, and the harmonics of the vibrations of its three atoms yield an optimized reflected wavelength of 698 nanometers.  Guess what color 698 nanometers corresponds to in the visible spectrum?  It's a bluish-violet, and even this is just barely still within the visible range.  This can actually be seen above.  Notice that there is some green reflected as well, which explains why water is aquamarine instead of a deep violet blue.

Yes, water is still transparent, meaning it can be seen through.  But so is a Jolly Rancher.  It's just not quite colorless.

Thus concludes my incredibly abusive and derogatory treatise on the color of water.  I hope you are offended.  Please refrain from sending horse heads, bags of flaming dog poop, or setting up burning crosses in my front yard.