Earth & Geography

Why Is This Ice Blue? Exploring An ICE CAVE!

Thanks to CuriosityStream for supporting PBS Digital Studios. What color is ice? Snow is made of ice, and it’s white. The ice most of us get out of our freezer is clear or cloudy. But glacier ice is different. It’s blue. But a blue you’ve never seen before. Until today. [MUSIC] Glacier ice may be the most beautiful blue in all of nature. Today, we’re going inside a glacier, to find out what makes this curious color. [MUSIC] I’d seen pictures of blue glacier ice before, but they didn’t do any justice to actually being there. This is an ice cave. It’s a cavity formed underneath tons of ice. Which means it’s the perfect place for us to figure out why ice is blue. [MUSIC] Look at that incredible blue. The sky is blue because light hitting the atmosphere is scattered and blue light is scattered predominantly, down into our eyes. But glacier ice is blue for a completely different reason. A much cooler one. Snow is also made of ice, and it’s definitely not blue.

Though it can be yellow. A single snowflake, viewed up close, is actually clear. But when snow piles up, it’s mostly air, and when light hits those air pockets, the faces of ice crystals act like a bazillion tiny little mirrors that scatter the full spectrum, white light, in every direction. Glacier ice begins its life as snow, but year after year, it’s squeezed by so much weight, the air bubbles between the crystals disappear. And without those air bubbles, white light isn’t scattered. But still, even a block of ultra-pure ice doesn’t show any color. It’s only when light travels deep into the glacier that the ice it can work its physics magic. Inside a glacier, the water molecules in ice are actually absorbing all light that isn’t glacier blue. And to understand how this works, we need to explore three ideas: wavelengths, frequencies, and overtones. Because light always travels at the same speed, the wavelength of light also tells us its frequency, the number of waves that cross a point in a certain time. Violet light? High frequency.

Red light? Lower frequency. It’s similar to how we think of sound waves: Higher frequencies [sound] and lower frequencies. [sound] Now you might not know this, but water molecules, even in ice, can vibrate, sorta like the atoms are connected by little springs. But like atomic pendulums, these littie springs only vibrate at a certain special frequency. If light at this special frequency comes along, if it’s in sync with the jiggling atomic spring, the molecule absorbs that energy and keeps vibrating, and that frequency–or color–of light is subtracted from the rest. Light that isn’t the right frequency passes right through. The thing is, a water molecule’s favorite vibration frequency is outside of the visible range.

So how can it have any effect on the colors we see? We can explain with some music. This is the A-string on a guitar [beat]. It’s tuned so that it vibrates back and forth 110 times per second, or 110 Hz. But we don’t just hear the 110 Hz sound when it’s played. We also hear “overtones”. A whole series of higher frequencies that depend on the instrument, how the string is plucked, a whole bunch of things that make a note on an instrument sound unique. The vibrations in water molecules can also be excited by “light wave overtones”: This light wave overtone has a higher frequency. Not every wave syncs up with water’s vibration, but some do, and they get absorbed. For solid water, one of those absorbed overtones sits right in the red/orange part of the spectrum.

When white light from the sun passes through the glacier, the red and orange frequencies are just right to be absorbed by the water molecules. They start vibrating. And what’s left when all the other light frequencies have passed through, is this beautiful blue color. That red and orange light isn’t absorbed very strongly, So it takes many many feet of ice to achieve this effect. That’s it: White light, minus red-orange light leaves us with this. Brilliant, blue ice! Sadly, ice like this is disappearing. This cave in Alaska will be gone within a couple years as the glacier melts and recedes up the mountain. Color in nature arises in many different ways, from the blue sky to butterfly wings, but this might be the only example where color comes from vibrations.

Clear liquid water absorbs sunlight in much the same way, and when you consider that about 70% of Earth’s surface is water, these good vibrations are the very reason our planet is a pale blue dot. Or maybe it’s a minus red dot. That’s the physics that makes a place like this so beautiful, and it’s hard to be blue with science that cool. Stay curious. Big thanks to CuriosityStream for supporting PBS Digital Studios. CuriosityStream is a subscription streaming service. They offer documentaries and nonfiction titles from some of the world’s best filmmakers, including some exclusive originals.

You can watch “Ancient Earth” a CuriosityStream original that shows what we think the world was like in the Permian, Triassic, Cretaceous, all the best eras. You can get unlimited access today. For our audience the first two months are free if you sign up at curiositystream dot com, slash smart. Use the promo code smart during the signup process. We’ll see you next week. Hey, before you click away, freeze! And check out this other video we made in Alaska about why glaciers move. It’s cool. I’m out of ice puns, sorry.

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