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Name: Diana Z.
Status: student
Age: 15
Location: N/A
Country: N/A
Date: Sunday, September 15, 2002


Question:
When a fat is placed on a piece of brown paper, why does it create a spot that is lighter and sometimes even translucent?


Replies:
Diana,

When light falls upon paper, some is transmitted, some is scattered, some is absorbed, and some is reflected. The fat connects the fibers in the paper with a liquid which can transmit by refraction (rather than scatter) light that falls upon it. As a result, the paper (if thin enough) seems almost transparent. In bygone days, windows in log cabins were sometimes made of oiled parchment so that they would allow more light to pass through them.

Regards,
ProfHoff 480


The reason for this effect is that the fat adhering to the cellulose fibers lowers the index of refraction of the cellulose and also fills in air voids, so that visible light passes through the bag with significantly less scattering. This effect is also important in the production of tracing papers for drafting, and art. These papers are also saturated with polymers that perform a similar function -- i.e. lowering the index of refraction of the paper fiber and filling in the air voids that scatter light, but without the "grease". You can also see a similar effect with "wax paper" which is more translucent than "regular paper".

Vince Calder


This is a result of "index matching". By this I mean that the index of refraction of the fat matches (or more nearly than air) the index of refraction of the fibers in the paper.

The index of refraction of a substance is the ratio of the speed of light in vacuum to the speed of light in the substance. Thus the index of refraction is almost always > 1 (the index of refraction of a perfect vacuum is exactly 1.0).

Light can do a couple of things when it strikes an object:

the light can pass through it
the light can be absorbed by it
the light can be reflected from it
the light can be scattered by it

The last case is the one that interests us here. The amount of scattering is affected by many things, such as the size of the particles (or fibers, in the case of papers) and the difference in the index of refraction between the particles and the surrounding medium. Normally when we look at paper the surrounding medium is air, with an index of refraction only slightly greater than 1.0. The paper fibers have a much higher index of refraction -- probably much greater than 1.5.

The fat also has a high index of refraction so that it nearly matches the index of refraction of the paper fibers and it reduces the scattering significantly. Now we only see the light that is reflected from the paper and much of the light that was formerly scattered back to our eyes is now transmitted through the paper.

Greg Bradburn



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