# Oil and paper opacity **URL:** \ **Category:** Factual Questions\ **Created:** [August 11, 2003, 1:32pm UTC](https://boards.straightdope.com/t/oil-and-paper-opacity/194372 "2003-08-11T13:32:29Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![optimystique](https://avatars.discourse-cdn.com/v4/letter/o/b38774/32.png) [@optimystique](https://boards.straightdope.com/u/optimystique)\ **Post date:** [August 11, 2003, 1:32pm UTC](https://boards.straightdope.com/t/oil-and-paper-opacity/194372/1 "2003-08-11T13:32:29Z")
I notice that when a piece of paper is smeared with some oil, it becomes translucent. Why is this so? ---
**Author:** ![Popup](https://avatars.discourse-cdn.com/v4/letter/p/94ad74/32.png) [@Popup](https://boards.straightdope.com/u/Popup)\ **Post date:** [August 11, 2003, 1:56pm UTC](https://boards.straightdope.com/t/oil-and-paper-opacity/194372/2 "2003-08-11T13:56:31Z")
This effect has been discussed before, but under the heading [Wet T-shirts](http://boards.straightdope.com/sdmb/showthread.php?s=&threadid=66540). **CalMeacham** wrote a long time ago: > [@](#): > > The secret to it all is index matching. Both the cotton fibers that make up a white T-shirt (assuming it’s uncolored) and the air are pretty transparent. The T-shirt appears white because you get reflection and scattering at each cotton/air interface. You can minimize this effect if you were to wet the cotton down with some liquid that has the same index of refraction as the cotton fibers. (The index of refraction is an optical constant that specifies how light is refracted upon entering the material. I(t also tells you the speed of light in that material). Even if the liquid doesn’t EXACTLY match the index of the cotton, it can still make the reflections smaller, rendering the shirt, if not completely transparent, at least translucent and (if in contact with what’s underneath) see-through. > > That’s what happens with a wet T-shirt. The water, having an index of refraction of 1.333, is still somewhat lower than that of cotton (which is somewhere in the 1.4s, I think), but it’s enough to make the shirt see-through. But you don’t see perfectly – only the things that are in contact. Of course, because the shirt is wet, more is in contact. That’s the appeal of a wet T-shirt contest. > > If you use oil on paper, the match is even better, but it can’t be perfect – in the first place, cotton fibers are birefringent, having different indices of refraction along different directions. In the second place, they put sizing in the paper to control the absorption of ink. They might have printing in either paper or T-shirts, and both may have fluorescent dyes added to make it appear “whiter”. All of this keeps either paper or a T-shirt from becoming completely transparent. Actually, even if these problems weren’t there, the paper or T-shirt wouldn’t become completely transparent, because it’s extremely hard to match refractive indices of liquids and solids over the entire visible spectrum. This leads to something called the Christianson effect, but I’m not going to get into that now. Personaly I prefer to imagine a wet T-shirt, but oily paper is just as good. ---
**Author:** ![Fear\_Itself](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/fear_itself/32/19637_2.png) [@Fear\_Itself](https://boards.straightdope.com/u/Fear_Itself)\ **Post date:** [August 11, 2003, 2:30pm UTC](https://boards.straightdope.com/t/oil-and-paper-opacity/194372/3 "2003-08-11T14:30:11Z")
> [@](#): > > _ **"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."** _ --Dr. Nick Riviera ---
**Author:** ![alimarx](https://avatars.discourse-cdn.com/v4/letter/a/e274bd/32.png) [@alimarx](https://boards.straightdope.com/u/alimarx)\ **Post date:** [August 11, 2003, 4:37pm UTC](https://boards.straightdope.com/t/oil-and-paper-opacity/194372/4 "2003-08-11T16:37:25Z")
> [@](#): > > \*Originally posted by Popup \* > Personaly I prefer to imagine a wet T-shirt, but oily paper is just as good. How about an oily T-shirt?