eli5:Why light can go through paper, but can’t through walls?
In: Physics
There are many facets to this. For starters, paper is like fabric – full of holes for light to pass through. You probably won’t find any walls with this attribute.
Secondly, paper is mostly made of cellulose. Cellulose is more or less transparent. The only reason that we can’t see through paper is because it is rough – the surface scatters light in all directions. If you sanded glass (google “ground glass”) you would get a similar white appearance, since the surface is now rough.
As for why some things are transparent and others are not, there are again many effects at play. Pretty much anything white will be white because it is granular or rough, or contains something granular. Paper, sugar, salt, paint, and so on. White is one of the more common colors for a pure substance to be. However, few substances are pure. Any elementary carbon will turn something very dark very fast because it absorbs light. A bunch of seemingly (not actually, but that’s way above my head) random molecules will absorb this or that specific frequency of light. If light is neither absorbed nor scattered, it passes straight through, like in a glass window. If it is only scattered or only a bit absorbed, then you get translucence, like paper.
There is one other case – reflection. Just as a smooth transparent surface makes for a see-through object, a smooth reflective surface makes for a mirror-like object. If either is rough, it appears white.
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