how come clear plastic sheets (folder wallet things to hold paper) are clear on their own but once you stack them up they become more opaque ?
In: Physics
They’re only *mostly* clear. If something is even 99.9% clear, then stacking a bunch of them together will block 0.1% more light with each additional one you add to the stack.
It’s the same reason that on a foggy morning, everything around you is always clear but it’s foggy further away. The air everywhere is *mostly* clear so when you’re looking at things close to you (through just a few “layers” of fog) everything is clear, but when you look down at the end of the block (through many more “layers” of fog), everything there is foggy.
Two things determine how you see objects that are even partially transparent: reflection and refraction.
Reflection is a direct redirection of light away from an object; some wavelengths being reflected more than others that are absorbed is how colors happen, for instance.
Refraction has to do with a more subtle bending of light as it moves through a fluid (gasses are fluids according to physics); how much is something we call the index of refraction. When something has an index of refraction that is very close to a certain medium, it is almost invisible when viewed through that medium (there is some plastic tubing that you can see pretty clearly outside of water but is nearly invisible inside the water, for instance).
See-through plastic sheets like the ones you’re talking about are refractive to about the same degree as air, and also are almost entirely non-reflective. But almost entirely isn’t the same as entirely. Stack those up and you get more and more reflection/absorption of light in the layers, until it stops being see-thru.
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