dimonium_anonimo t1_iuhxl5b wrote on October 31, 2022 at 1:52 PM

It depends (a little bit) by what you mean by particles but significantly more by what you mean by stacked, and also what material the particles are.

Let me give you an example. A meter stick is a 1-meter tall stack of atoms. Atoms are waaaaaay smaller than microscopic. You can see a meter stick, right? But, if you look end on to a meter stick, you see that it is, maybe an inch wide by a half cm tall. So there are already quintillions of atoms just in every layer of the stack.

So maybe you mean a single particle stacked on another single particle... All the way up to 1 m tall. This would be a very very unstable beam. Like balancing a km long pencil probably. But what kinds of particles. If they are just barely too small to see with the naked eye, then even two of them should become just barely visible.

If instead, they are significantly too small to see with a naked eye, then stacking them 1 wide will never become visible. Our eyes can't distinguish things smaller than a certain visual angle, so if the particle stack itself is smaller than that, we won't see it.

There are some notable curiosities tangentially related. Gold can actually form an opaque layer at only a few atoms thick. That's why gold plating is so useful, and gold leaf exists. It's not a 1 m tall stack, but you could say it's a few nanometers tall if you're looking at thickness, or you could say it's however wide and long the object being played is, but then we're going back to the particles being stacked in two different directions that is now visible because obviously it would be.

Fitzanor OP t1_iuiyy3e wrote on October 31, 2022 at 6:06 PM

Thank you for your in depth answer. I'm not a native speaker and "stack" was the wrong term I guess. I meant a pile or something ( what we call " un tas" in french). Like if I take billions of too-small-to-be-seen particles and dump them together.

dimonium_anonimo t1_iuj5bmt wrote on October 31, 2022 at 6:49 PM

Right. If the particle you're looking at has a very similar refractive index as air (or if the pile is in water, then the refractive index of water. Whatever medium your pile is in) then you are not likely to see any visible aberrations at all. This needs to be close like withing a millionth of a percent.

If your particle has a significantly different refractive index, but is still mostly transparent, then you are likely to see it clearly. Even if a contiguous mass would be near transparent, every time the light passes from one substance to the other (and a pile like this is likely to have millions of such interactions) then it will scatter a bit of light every time. You will see this most likely as a white powder.

The only other option is that the particle is not transparent. It either absorbs or reflects most visible light, in which case it will look like any other pile of stuff. It will take on the visual properties of that stuff. Except reflection might end up looking more scattered or matte than normal. As an example, a really shiny metal that you could see your reflection in when atomized is likely to lose the perfect surface finish that gives it that specular reflection where an image can be made out. It is going to reflect the light in all directions equally so it will just look like a pile of silver dust.