# How do photon particles travel through glass?

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Been studying science in college for about 2 years and this simple question has me questioning my own IQ. I understand how light travels through different mediums but photons are particles right? Actually physical particles that can travel through solid blocks of glass?

I dont know if Im just stupid or my teacher doesnt care, this question could keep me up at night.

In: Physics

To a photon, that glass isn’t solid. In fact, it’s mostly empty space. Atomic and particle physics govern the rest, including how different atoms/molecules absorb different types of light/photons. For ELI5 purposes, reorient yourself from the large scale (the you-sized scale) to the incredibly small scale (the photon scale).

Photons are not *only* particles, they are also waves. You’ve probably heard that most of an atom is empty space. That’s *sort of* true. Electrons don’t “orbit” the nucleus in the sense of a tiny particle whizzing around the nucleus – electrons are *also* also waves. They exist in a “cloud” of probability around the nucleus where they *can* be in any position and behave in many ways as if they are in *every* possible position.

A photon will pass through these clouds of probability and interact with the electrons there.

Photons give electrons energy, energizing the electrons and moving them into a higher state around the nucleus. The more energy a photon has, the higher the energy state of the electron after it absorbs a photon. For quantum mechanics reasons, electrons can’t exist halfway between states. They are either all the way into the higher state or not at all. Think of it like an elevator that can’t stop halfway between floors.

Also for quantum mechanics reasons, these energy states can only have so many electrons in them. If the energy state is full, an electron *cannot* move into that state.

If a photon doesn’t have enough energy to move the electron *all the way* into an *empty* energy state, the photon cannot be absorbed by the electron and it will pass right through that electron cloud.

Glass and water and stuff that is transparent to visible light is transparent because the electrons are all arranged to fill up the energy states so that no photons in the visible part of the spectrum have enough energy to move any of the electrons all the way into the next empty state. Therefore, none of those photons can be absorbed and they go right through.

You’ve got 2 misapprehensions here. First, photons are not just particles. They are vibrations in the everywhere-permeating electromagnetic field. Second, on the scale of photons, glass is far from solid. Depending on your interpretation of quantum mechanics, the photon might not see electrons as clouds of probability density, but as point particles separated by real distances. Empty voids that they can pass through.

There are different models for how light actually work, but without going into that much detail.

Light goes on and on until it hits on of the atoms of the glass just like it would hit something black, like soot and it gets absorbed.

When it hits soot it gets absorbed and that’s it. Sooth gets a bit warmer.

When it hits glass it also gets absorbed, but a new light particle (photon) gets emitted soon after and goes on until it hits another atom and the process repeats.

This is how light moves in all amtter – it gets absorbed and re-emitted all the time. This takes some time and is a reason why light in matter travels slower than in vaccum.

Solids (and in fact all materials) are a collection of atoms.

Atoms are a tiny nucleus, surrouneded by some electrons.

Electrons can absorb photons, but they do so by following quite varied and specific rules, and when they absrob them, they might release it again, perhaps back in the same direction, or some random direction, or in chunks, etc.

So instead of thinking of a material as all-the-way ‘solid’, think of it as more like a mostly empty gauntlet or obstacle course, with gaps and bars and mirrors etc etc. But remember that there are perhaps trillions of these little obstacles in the way, arranged in all directions in the 3D space the material occupies.

Depeding on the type of light, and the shape of this obstacle course, some photons will pass through, some will be reflected, some will scatter in various directions, and so on.

Now, this ‘obstacle course’ is basically me handwaving a whole lot of quantum mechanics. The obstacles are things like electron energy levels and whether electrons are free or not, etc.

This 3blue1brown video playlist is the best intuitive explanation of light propagation I’ve seen. A bit more than ELI5, but perfect for a college student.

Have you ever put your fingertip on a source of a concentrated light? Try putting a finger on your phone flashlight. The photons actually travel more or less through the solid matter. Whether it will fully pass through depends on how thick and compressed the matter is.

Biggest wrong assumption you’re making is saying photons are particles. They’re not. Sure, sometimes we choose to *treat* them as particles because it works as a way to understand how they behave, but they aren’t. An actual particle would have mass. Photons don’t.

They’re “compartmentalized” waves. If you could make a wave on water that never spreads out, but remains as a sort of coherent single crested wave, that’s closer to what a photon is. You could treat that as a particle too, one that bounces off obstacles and such. But this hypothetical wave is not actually a physical *thing* that exists. It’s just the water surface being warped because of kinetic energy, like any wave in water is. A photon similarly isn’t a physical thing that exists, it’s just a fluctuation of electromagnetic energy.

This doesn’t answer how it travels through glass, because I have no clear cut answer for that, but it hopefully helps along with the other answers to understand better what photons actually are.