Why does grease make paper semi-transparent?

The subject header tells it all. Mayonnaise and spilled oil have done it for me.

Also, I think I read somewhere that Med. and Renaissance painters put such greased papers over their windows for the particular kind of lighting they needed…

Why does grease make paper transparent?

Thank you very much. Sorry for not searching.

No problem. I linked the thread not just because there had been a thread on it but because CalMeacham provided the definitive answer. He’s an optics expert.

Let me point out that this phenomenon gives an excellent way of comparing the luminosity of two light sources. Put a grease spot on a piece of paper and hold it between the two sources. When the the illumination on your side is greater, the spot appears dark and when it is greater on the other side, it turns bright. When th luminosities are the same the spot disappears. Using this simple device, I was able to determine that a 23 watt CF bulb that I had bought as a reading lamp had only about 40% of the luminosity of the 100 incandescent bulb it was supposed to replace. It had been advertised as being equivalent to a 100 watt bulb, but wasn’t in the same ball park.

I’d like to add one additional thing to the answer given in the previous thread, which also suggests an experiment

Paper is often made with white pigment to improve its opacity (making it look whiter). Often, the pigment is titanium dioxide, which is among the best choices precisely because its index of refraction is so high, thus making it an excellent scatterer of light. I haven’t done the experiment, but it should follow that the brightest high quality white paper will be harder to make transparent with grease, since the index of the grease, while close to that of cellulose, will still be very different from that of titanium dioxide.

This would be true only an a fairly approximate way. It would depend on the angular size of both of the sources from the point of view of the paper, and also on how completely Lambertian the paper is, and how close the grease spot is to the line between the far source and the observer’s eye, and on the index of refraction mismatch between the grease and the paper solids, and finally on the absorption coefficients of the paper solids and the grease.

Not true – this is the basis of the Grease Spot Photometer, which has been in use for quite a long time.





I actually looked on both sides and got the disappearance at the same spot. I don’t see how any of those confounding variables could have made a difference. Incidentally, I was using identical lamps with the same shade.

I’m sure the simple method of matching the brightnesses can’t be quite correct.

If the directionality of the illumination is controlled on both sides, the situation improves. Note that the SEI photometer takes charge of the angular distribution of incident light on both sides. So does the photometer at the kenyon.edu site.

Also, note that the grease spot photometer at kenyon.edu and the descriptions at http://kr.cs.ait.ac.th/~radok/physics/l2.htm and at http://www.nature.com/nature/journal/v40/n1025/abs/040174d0.html require separate readings taken to make the spot disappear as seen from one side, and as seen from the other side, and require some kind of average between the two. The SEI photometer could avoid doing this because it is always using the same incident light (which is built in). Other grease spot photometers all seem to include mirrors to provide for viewing both sides of the paper.

To quote http://kr.cs.ait.ac.th/~radok/physics/l2.htm,
“If both sides of the paper are equally brightly illuminated, the greased spot cannot stand out of its environment, it becomes invisible. (Actually, it does not become completely invisible; it retains a minimum of visibility, but this minimum appears to be equally strong on both sides of the paper.) In order to see both sides next to each other, you place behind the screen an angled mirror, so that the screen lies symmetrically with respect to the two mirrors. You change the distances of the sources of light to be compared with each other until the greased spot has on both sides minimal visibility. The distances yield then the ratio of the intensities of the sources of light.”

Several things would go wrong if you tried to make this work as viewed only from one side.

First, the angular dependence of the way the paper scatters light is going to influence the apparent brightness of the source on the same side of the viewer, and this is going to interact with the angular size and position of the source. Is it construction paper, or is it the paper magazines are printed on, or somewhere in between? If you temporarily replace the paper with a mirror, would you see the source in the same location as the grease spot?

Second, the grease spot can change the reflection of the light on the same side as the viewer. Is it slick enough to reflect light specularly? If so, given the viewer’s eye location, does the reflected view include the source?

Third, what the grease spot is doing primarily is filling voids in the paper with something that’s a better match for its index of refraction than is air. This makes it not just more translucent, but actually more transparent. With the right paper and the right grease, one might be able to read printed words a very short distance behind the paper. So, since the spot increases the degree to which rays can travel nearly straight through, it will make a difference if the distant source is along the line of sight from the observer through the grease spot.

The descriptions of the classic grease=sopot photometer I’ve read require readings from both sides, so I’m not going to defend single-side reading.

As for what the grease spot does, read my response on the other thread cited above. It’s really tough to make things transparent with index-matching even when you have complete control over the index – i speak from experience here. It’s worse when the medium itself (cellulose fibers) is itself birefringent. It’s true that the early colonists tried to make crude windows with greased paper. That the attempt didn’t last beyond a year proves how poor it was. You can sometimes read a very short distance beyond the paper, but that’s really short. For practical purposes, and the distances the photo,meters worked over, they were translucent rather than transparent. the devices had their faults, and there were undoubtedly corrections that could be made, but they worked pretty well for an invention from Bunsen’s time, and were used into the 20th century. More than a “fairly approximate” device.

OK, if we are talking about reading from both sides and grease spots that are still just translucent and not transparent, I could imagine the devices are better than “fairly approximate”. My characterization is too ungenerous. It’s probably a good method and certainly a clever one, especially for its day. Maybe excellent…