Re: When fat is rubbed on paper, why does it go transparent?

Thanks for the question, John.

It is interesting that the paper can become translucent when it has certain fats or oils rubbed on it. The reason for this translucence is because the index of refraction (or refractive index) of the fat is very close to that of the cellulose fibers that make up paper.

The index of refraction is a material property that indicates how fast light travels through a substance. The index of refraction is found by dividing the speed of light in a vacuum (3 *10^8 m/s) by the speed of light in the material. Therefore, all values of the index of refraction are greater than 1.0.

When two materials have very similar indexes of refraction and are allowed to make intimate contact with each other (intimate contact in this context requires that there is no air between the fat and the paper), the materials will appear to be translucent. The fat and paper fiber indexes of refraction are both around 1.5.

I have taken advantage of this phenomenon in a recently-submitted journal article where my colleagues and I placed a mineral oil on contaminated paper to render the paper fibers translucent and allow us to characterize the contaminants that were hidden within the sheet of paper.