Why does grease make paper transparent?

As i sit here with my sausage and hash brown barmcake, i wonder: How could the grease that’s dripping off, make my pad of paper seethrough?:confused:

I might be completely wrong here, but I always thought it was because the grease filled the microscopic holes in the paper, kinda like millions of tiny bubbles?

Not far off. I’ve answered this one before. Paper is mainly cellulose fiber. They’re separated by air, so if you fill in those gaps, you’ll reduce the scattering at the many air-cellulose interfaces. It would be ideal to fill it in with a liquid whose index exactly matches that of the cellulose, because then (in an ideal world) the fibers become invisible, and the paper would be completely transparent. But you still get significant reduction in the scattering even with only an approximate match. So water-wet paper is translucent. Oil-wet paper matches even better, so it comes closer to being transparent.
Unfortunately, I doubt if you’d ever get perfectly transparency. For one thing, there are other things in paper – brightening agents, coloring agents, sizing (to keep ink from running on the paper – otherwise it would be like a paper towel). In addition, not all of your fibers are going to be optically equivalent, and there’s going to be some index variation. In fact, there’s index variation within a single fiber, since cellulose is actually birefringent, and has a different refractive index parallel to the long direction of the fiber than it does perpendicular to that direction.
T-shirts are made from cellulose fiber, too, and when you get them wet they turn translucent, and effectively transparent for objects sticking to the cloth. Wet T-shirts – Just another way science brings us benefits.

Dr. Nick: Hi everybody!

Homer+Bart: Hi Doctor Nick!

Nick [to Homer]: Now there are many options available for dangerously underweighted individuals like yourself. I recommend a slow steady gorging process combined with assal horizontology.

Homer: [pensive] Of course.

Nick: [points to a chart] You’ll want to focus on the neglected food groups such as the whipped group, the congealed group and the chocotastic!

Homer: What can I do to speed the whole thing up, Doctor?

Nick: Well…be creative. Instead of making sandwiches with bread, use poptarts. Instead of chewing gum, chew bacon, heh…

Bart: You could brush your teeth with milkshakes!

Dr. Nick: Hey, did you go to Hollywood Upstairs Medical College too? And remember, if you’re not sure about something, rub it against a piece of paper. If the paper turns clear, it’s your window to weight gain. Bye bye, everybody!

Do I infer, then, that all of the white color in paper is due to light scattering rather than the paper’s actually being white?

There is money to be made by the person who figures out what liquid has an index that exactly matches that of the cellulose of T-shirt fibers, or produces a synthetic fiber capable of an ideal match. We could go from “effectively” transparent to “hey, where did the T-shirt go” transparent.

Do I infer, then, that all of the white color in paper is due to light scattering rather than the paper’s actually being white?

Yep. Look at the fibers under a microscope – they’re transparent.

Yeah, but see my comments on why paper isn’t transparent.

Besides, liquids that more closely match the refractive index are likely to be icky, non-edible or potable, and possibly dangerous. Water is user-friendly. And it dries with no residue. Or you can lick it off.


I hate to ask this, but what are the real-world applications for studying birefringence in wood pulp? I’m sure they’re out there, but somehow all I can think of is the idea of a beam splitter made out of oak.

What, you never heard of the Wood Lens?



Actually there is no such thing as “white.” It’s just scattering.

Shave a clear block of ice to make a pile of opaque white snow. Grind a big transparent halite crystal to make “white” salt grains. Finely ground glass looks white even if the original glass was deeply colored. Of course you can go too far with this: if the particle size gets way smaller than the length of light waves, then the pile of powder will be transparent like aerogel.

To get rid of the “white” just reduce the scattering by applying an index-matching liquid. The sidewalk turns darker when wet. So do washcloths and old chalky painted houses. Spray some flurocarbon liquid on a letter to read the writing without opening it, and after it evaporates, only the ozone layer knows that this ever happened.

But T-shirts are already transparent. Oh yeah, you humans can’t see millimeter waves or even longwave IR.

Swat teams carry cooled-array FLIR scopes which can see through wood and sheetrock walls, but they also make clothing seem to vanish. Or so I’ve been told. The fine grain “white” particles in those materials is too small to scatter the longwave light.

Gotta love it.

Generally true, but scattering can also be created by opaque reflective substances such as metal. “Brushed” metal surfaces look white, and applying liquid won’t change the appearance.

Infrared cameras do see through thin clothing. I believe SONY had to pull one of their video cameras off the market because its “night scene” mode had this effect.

For extra white paper I do believe they can add extra scattering agents such as Titania. Too lazy to look it up and google is down at the moment