Prepare to be mesmerized by the video above. The folks from Firefight Films piloted a GoPro-equipped quadcopter drone through massive glacial ice caves in Alaska, and the result is stunning. It’s “cool” in both senses of the word, eh?
After I broke free from the slack-jawed catatonia (videos like this do that to me), one thought stuck with me: Why is the ice so blue?
Water is almost entirely transparent to visible light. Like, amazingly so. That’s the reason that life exists in the first place, because photosynthesis could occur near the surface of prehistoric (and modern) oceans. But it’s not totally transparent to visible light. It selectively filters out red stuff, leaving just the blues behind. B.B. King would like that.
So what gives? It’s not because of the reason that the sky is blue. That’s due to a phenomenon called Rayleigh scattering, where air molecules scatter short wavelength (blue) visible light into our eyes (it’s also why sunsets are red, but I made a whole YouTube video about that if you want to know more).
Nope, ice and water are blue thanks to a completely different, and more complex, bit of cerulean science.
Water, even when frozen solid, isn’t rigid. Like any molecule, it vibrates. The hydrogens are ever-so-slightly oscillating around the central oxygen, stretching those covalent bonds to and fro. Since a water molecule has three atoms (N=3), that means it has 3N-6 primary dimensions of vibration (3 total). Check ‘em out below:
Just at ‘em, wavin’ their hydrogens in the air like they just don’t care! You can easily mimic these aquatic dance moves in front of your bedroom mirror, or at the club, just be sure to credit me next time you bust out the “water shuffle”.
And just like a vibrating string, a vibrating molecule can emit overtones. Overtones in molecules are kind of how a guitar (or violin, or banjo, or any stringed instrument) can make harmonics, only with a hefty spoonful of Fourier transforms added. “Fourier transform” may bring back nightmares of sweaty-palmed calculus exams, but in truth they are one of the most elegant principles in math, underlying everything from mp3’s to Homer Simpson’s face.
These vibrating water molecules can absorb energy at very particular wavelengths. The physics behind this absorption gets complicated real quick (you can drink deeper here if you’re so inclined), but you can observe a similar phenomenon right in your kitchen. You know how your microwave heats up food thanks to the molecular shaking induced by long-wavelength radiation? You didn’t know that? Well, that’s how it works. Except when it comes to blue water, instead of long-wavelength microwaves vibrating entire water molecules, we have shorter wavelength radiation sending just the arms of water molecules into harmonic vibrations.
It just so happens that, thanks to all those combined overtones and disco-dancing hydrogens, water absorbs a tiny bit of electromagnetic radiation around 698 nanometers in wavelength. That just so happens to be red light! (Water also slurps up plenty of other wavelengths across the spectrum, but very little of that happens in the visible range):
Liquid or solid, water shines azure, stripping visible light of its reds, and leaving only the blue hues behind to be reflected back to our landlubbers’ eyes thanks to microscopic particulates.
There’s always something to learn, even when it comes to water, a chemical we think know so well. Goes to show, even the clearest of views can unlock curiosities when we look deep enough…
(Bluest of blue image of Crater Lake, via Wikipedia)