Why the Heck Is Glass Transparent?
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If the earth was one giant atom, its nucleus could fit inside a baseball stadium!
Everything outside the stadium—the rest of the planet?
That's where the electrons live. In a sort of wave-y, quantum-y cloud.
The stuff that makes up stuff doesn't contain much actual stuff. Huh. But if an atom is
just a miniscule nucleus surrounded by a wave-y, quantum-y cloud of mostly nothing, kinda makes you
wonder: Why doesn't light woosh right through the atoms in bricks, or steel, or chocolate ice cream?
Why aren't you and I transparent?
[MUSIC]
Hey smart people, Joe here.
So, why aren't we transparent? Well, we are.
If you're an x-ray! Our bodies just aren't transparent to visible light.
Of course, visible light and x-rays are both just different forms of electromagnetic radiation,
with different wavelengths and energies. So what's the difference?
Well, have a seat, because glass is in session (BOO)
Glass is transparent to visible light. If we zoom down to the atomic level,
we see glass is made up of a bunch of silicon and oxygen atoms. Same as this stuff,
sand! When that sand was melted down into a liquid, those molecules left the nice,
perfectly repeating crystal shape they were living in, and went wild. Until we cooled
them down really fast, and they froze in place, in a sort of organized jumble.
All those atoms are surrounded by wave-y, quantum-y electron clouds.
But the electrons around a nucleus can't be just anywhere. They live on specific energy
levels - think of them as different distances from that tiny nucleus.
When a photon comes by, with exactly the right amount of energy,
it gets absorbed, bumping an electron to a higher energy level.
But if that photon doesn't have just the right amount of energy it passes right by. Woosh!
Imagine I'm an electron. I'm hanging out here, at a low energy level. I want to move up there.
To higher energy levels. To make it happen, I have to have just the right amount of oomph
in my jump. Too little, and I don't make it. Too much, and well… oops. Just right...
For the particular atoms that make up glass, the energy levels are so far apart
that visible light doesn't have enough energy to boost those electrons up to the next level.
That's why visible light passes right through! But photons of UV light do have the right amount of
energy to power up those electrons, and they get absorbed. Which is why glass is opaque
to most UV! And why it's hard get a sunburn through a window.
How transparent something is depends on this relationship between light energy and an atom's
electrons. Different elements have different energy requirements for their electrons to
absorb light. Like how when visible light hits my atoms, it's absorbed. Some light might get through
a few top layers of skin cells, but within a few millimeters all the photons get gobbled up. That's
why I'm not transparent. But hit me with higher energy waves, like X-rays, and I am transparent.
Glad we cleared all that up (BOO)
But, thinking about how atoms are wave-y, quantum-y mostly empty clouds makes me wonder
something else: Why am I even here? Why aren't the mostly empty atoms in my feet passing right
through the mostly empty atoms in the ground, sucking me into Earth's superheated iron core?
Why can I sit on a chair, kick a ball, or smash those like and subscribe buttons?
Why can I touch anything?
Let's say I would like to boop this snoot. My finger—or “booper”—and
the snoot are both made of about a gajillion atoms,
give or take a squadrillion. And all those atoms are surrounded by negatively charged electrons.
As the two objects get close enough together, the negatively charged electron clouds at both
surfaces repel each other, thanks to what's called electrostatic repulsion.
The actual boop itself, the sensation of touch,
is caused by an actual force from this repulsion, acting on pressure-sensitive nerves in my skin.
Kind of like how we're never really aware of the atmosphere until there's wind pushing against us.
Touching something doesn't really mean decreasing the distance between me and something else to
zero, it's just getting my atoms and that other object's atoms as close as the electrons—and
physics—will allow. Of course, there is one more way that electrons can interact.
It is actually possible for two negatively charged electrons to occupy the same
quantum-y cloud energy level… as long as they have opposite directions, or signs,
for a property called “spin”. And sometimes electrons in two
different atoms can be squished close enough that their wavy-ness even overlaps! That's
the reason covalent chemical bonds exist, which is pretty convenient.
Like the ones in your body. All those molecules that keep you alive, full of atoms,
absorbing visible light photons. And just like the bonds between oxygen and silicon,
in this camera lens, in the fiber optic cables of the internet, even in the glass of the
screen between us right now, that are letting photons of visible light pass right through,
which I hope is making this touching bit of science just a little bit clearer.
Stay curious.
