(narrator) Thank you to Curiosity Stream for supporting PBS. If the Earth was one giant atom, its nucleus could fit inside of a baseball stadium. Everything outside the stadium, the rest of the planet? That's where the electrons live, in a sort of wavy, 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 wavy, quantum-y cloud of mostly nothing, kind of makes you wonder, why doesn't light just whoosh right through atoms and bricks or steel or chocolate ice cream? Why aren't you and I transparent?

(Describer) A man *is* transparent. Title: It's Okay to Be Smart The man becomes *not* transparent.

[pizzicato string music]

Hey, smart people. Joe here. So why aren't we all 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.

[audience booing]

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 that they were living in, and they 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 wavy, quantum-y electron clouds, but the electrons around a nucleus can't be just anywhere. They live on specific energy levels. You can think of them as different distances from that tiny nucleus. Now, 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-- whoosh.

(Describer) In a video game...

Imagine I'm an electron. I'm hanging out here at a low energy level, and I want to move up there to a higher energy level. To make it happen, I have to have just the right amount of oomph in my jump. Too little... ugh...

(Describer) He doesn't reach the level.

and I don't make it. Too much, and, well, oops.

(Describer) He jumps too far.

Just right... [upbeat chime]

(Describer) He jumps to the level.

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 just 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 pretty hard to 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 those 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.

(Describer) Title: It's clear you need new jokes.

[audience boos]

But thinking about how atoms are these wavy, 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 or kick a ball or smash those like and subscribe buttons? Why can I touch anything? Let's say I'd like to boop this snoot.

(Describer) ..on a dog.

My finger, or booper, and the snoot are both made of about a gajillion atoms, give or take a squadrillion, and all of those atoms are surrounded by negatively charged electrons. As the two objects get close enough together, the negatively charged electron clouds of 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 the 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 together that their waviness 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 that's between us right now, that are letting photons of visible light pass right through and which I hope is making this touching bit of science just a little bit clearer. Stay curious.