Why does grease or oil make paper more translucent?
The paper transmits light because oil permeates the pores of the paper and index matches to the cellulose. You don’t get perfect transparency for various reasons: (1) Typical oil isn’t a perfect match for cellulose. (2) Typical paper contains not just cellulose but other junk. If you match to one, you can’t match to the other. (3) What’s worse is that the cellulose fibers are hollow and you’ll never get oil into the cores. Therefore there will always be some scattering off the cores. n optics and fiber optics, an index-matching material is a substance, usually a liquid, cement (adhesive), or gel, which has an index of refraction that closely approximates that of an optical element or fiber, and is used to reduce Fresnel reflection at the surface of the element.
