[MUSIC] 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 la cierma, gic.th e water molecules in ice are actually absorbing all light that isn't w thisgl acwoierkr s,bl wuee .ne ed to explore three ideas: wavelengths, frequencies, Beancad usove erlitoghnet s.al ways 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.

r mo[sleoucundle]s, 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.

As white light passes through the glacier,red and orange frequencies are just right to be absorbed by the water molecules.

They start vibrating.

And all the other light that passes through, leaves us with this beautiful nd orablnguee cliolorgh.t isn't absorbed very strongly, So it takes many many feet worth 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 nt ain.

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.