Wax Rubbed on Paper: Why Paper Becomes Translucent

[khil_phys]

Why does paper become translucent when wax, butter or oil is rubbed on it?

 

[jbriggs444]

I have always assumed that it is because the wax (or oil or whatever) more closely approximates the refractive index of the paper than the air that would otherwise separate the fibers in the dense mat that we call "paper".

The amount of reflection and the amount of refraction taking place at an interface is going to depend in large part on the difference in the refractive indexes of the materials on each side of the interface.

 

[sophiecentaur]

Interesting question and I'm surprised this forum isn't full of variiations on it.
I, also would think it's to do with refractive index but it could do with Total Internal Reflection. When light arrives at dry paper, there is no internal reflection (air having a low refractive index) so tends to get absorbed by the fibres. When there is water or grease, the light could be totally reflected at the interfaces and find itself penetrating much greater depth of the material by a series of reflections. I imagine there will be some fibrous substances that would show this effect less and some would show it more.
I did have another idea and that is to do with the surface, when it's wet. It's much flatter and would not scatter the light so much, possibly allowing more light to cross the interface (on the way in and out) - more specular.

 

[pgardn]

^^^^^^^

Exactly what I was thinking about the scattering and a lens effect when the oil gets in between all the fibres that hold little oil "window panes". So you have essentially made a smoother product that allows light to penetrate.

On a biochemical level lots of paper products are extraordinarily good (paper towels) at holding oil which one might not expect based on the fact that they are mostly carbohydrates which are polar. But processed cellulose (and unprocessed cellulose as well) take on some extremely hydrophobic characteristics as well so it can sop up oil and water. Kind like a solid soap in one sense. Its an amphipathic product.

 

[khil_phys]

Why isn't there a theory on this? I mean, I was "taught" this 7 years ago in primary school, and took it as a fact. This is most certainly an everyday experience. What might be the hurdles in explaining this phenomenon?

 

[sophiecentaur]

Why isn't there a theory on this? I mean, I was "taught" this 7 years ago in primary school, and took it as a fact. This is most certainly an everyday experience. What might be the hurdles in explaining this phenomenon?

You'll have to do some research of your own and make a name for yourself as an expert!

 

[mikeph]

I think the idea is that the fibre/air refractive index ratio is very high so light is highly refracted, and scattered, when it hits the dry paper. The wax replaces the air in the paper, and the fibre/wax refractive index ratio is far closer to unity so the incident light tends not to undergo so much refraction, and can be transmitted through the material without being significantly scattered.

 

[pgardn]

Why isn't there a theory on this? I mean, I was "taught" this 7 years ago in primary school, and took it as a fact. This is most certainly an everyday experience. What might be the hurdles in explaining this phenomenon?

Took what as fact?

 

[khil_phys]

Took what as fact?

This very phenomenon.

 

[pgardn]

Well I imagine quite a few people have noticed the same phenomena.

But your question was why does oil cause paper to appear translucent. People are discussing the why, not the apparent fact that the paper does appear translucent.

Its a very good question. Its one of those types of questions that lots of people notice but never contemplate the why part of the observation.