Why does Paper get transparent if you rub it with fat?
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Reflections basically happen when light goes from one material with some index of refraction to another material with a different index. Its just a property of the material.
Paper looks opaque because it has lots of air/fiber surfaces for light to bounce around on. Unlike something truly opaque like say a brick, light still passes through the paper. It’s just bounced around so much the color washes out and sharp lines get blurred.
Fat, or oil or water fills in the gaps. Oil and paper have closer indices of refraction than air and paper. The light is less likely to bounce around and more likely to just pass straight through. That lets you see stuff on the other side. At least if it’s close enough to not get blurred too much.
This is because when fat is absorbed into the paper’s pores, the grease-stained part of the paper—which is normally white due to the scattering of light that shines through it—allows less scattering, and the light passes through it, appearing darker in color and translucent.
First let’s look at oil. Have you ever watched Mom or Dad cook spaghetti?
If they cook it in a pot of water, you might have seen some big droplets of oil floating on top. If you look at the bottom of the pan, you’ll notice that the oil turns the light above the stove into shapes that match the droplets.
That happens because the droplets of oil act like magnifying glasses. That means that the droplets bend the light – like a magician making a balloon animal, but a little bit simpler – the droplets don’t know how to make animal shapes so each droplet just makes the light into a shape like itself.
Now let’s have a think about paper. It looks pretty solid, but if you tear it, you can see that the edges are fuzzy. That’s because paper is made of many, very small, strings. Have you ever seen the lint trap of the dryer before Mom or Dad cleans it? Paper is like that blue or grey or sometimes red fuzzy stuff on the little screen, except it’s pressed flat and stuck together to form nice sheets.
So the reason we can’t see through paper normally is because there are so many of those little fuzzy hairs, called fibers, which basically block our view so we can’t see what’s on the other side.
Now what happens when we smear oil on paper? Of course the oil loves it! The oil thinks it’s just fantastic and flows around to hug all of the cute little fibers in the paper!
Remember we said earlier that the droplets of oil on the cooking water like to bend light into shapes like the droplets? Well now the oil in the paper does the same thing.
Since the oil is surrounding and hugging all of the fibers, it also bends light so that the light can move around and hug all of the fibers. Then, the light continues on its way, out the other side of the paper.
In summary, the paper is like a forest of itsy bitsy trees that light doesn’t know how to get through, so it just turns back and we can’t see through the paper. Oil likes to go and hug all of the trees in the forest so the oil, knowing where all the trees are, also knows the path to get through the trees. So, when light approaches the paper-forest with oil, the oil says “Hey, light! Don’t worry, I’ll help you find your way!” And the oil does, and light can then go through the paper forest, bringing with it the picture for you to see of what’s on the other side.
Why are you rubbing fat on paper? ELI5.
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