Why Is Glass Clear?

If the Earth were one huge atom, its center could fit inside a baseball stadium! Everything beyond the stadium—the remainder of the planet? That’s where the electrons reside. In a kind of wavy, quantum cloud. The substance that makes up objects doesn't really contain much material. Huh. But if an atom is just a tiny nucleus surrounded by a mostly empty cloud of wave-y, quantum-y stuff, it makes you wonder: Why doesn’t light just pass through the atoms in bricks, steel, or chocolate ice cream? Why aren’t we see-through? So, why aren’t we see-through? Well, we are. If you're an x-ray! Our bodies just don't let visible light pass through. Of course, visible light and x-rays are both forms of electromagnetic radiation, with different wavelengths and energies. So what's the difference? Well, let's dive into it! Glass is transparent to visible light. If we zoom down to the atomic level, we see glass is made up of silicon and oxygen atoms. Same as sand! When that sand was melted into a liquid, those molecules left the nice, repeating crystal shape they were in, and went wild. Until we cooled them down really fast, and they froze in place, in an 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 - 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 to get a sunburn through a window. The transparency of a material depends on the 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 absorbed. That’s why I’m not see-through. But hit me with higher energy waves, like X-rays, and I am transparent. Glad we cleared all that up! But, considering how atoms are 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.

Stay curious.