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. 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.
