So, for some reason, my ten year old video on the Barkhausen effect went viral on YouTube the past few weeks, and brought me quite a few new followers there. I thought I’d do a short video to say “thanks for following” to folks and try out a demonstration that I’ve been meaning to get around to for a while related to invisibility.

The video, about the so-called oculus mundi stone, is embedded below.

Oculus mundi (“eye of the world”), also known as hydrophane opal, is a stone that is largely opaque under normal circumstances, but when it is submerged in water, it becomes quite transparent. As noted in the video, the reason for this is that the stone is highly porous in a dry state, and all those pores strongly scatter light that shines upon them, making the stone opaque. The same sort of thing happens with milk, which is a bunch of fat molecules in water: the fat molecules make the light bounce around and generally scatter it away from the liquid. When dumped in water, the pores in oculus mundi become filled with water, and we get a rough “index matching” situation: water has a refractive index closer to opal than air does, so light gets scattered less by the water-filled pores.

I came across reference to oculus mundi while writing my upcoming invisibility book, because Isaac Newton used it as evidence for his hypothesis about the relation between opacity of a material and its microscopic structure. In his own words, from Opticks:

But farther, that this discontinuity of parts is the principal Cause of the opacity of Bodies, will appear by considering, that opake Substances become transparent by filling their Pores with any Substance of equal or almost equal density with their parts. Thus Paper dipped in Water or Oil, the Oculus Mundi Stone steep’d in Water, Linnen Cloth oiled or varnish’d, and many other Substances soaked in such Liquors as will intimately pervade their little Pores, become by that means more transparent than otherwise; so, on the contrary, the most transparent Substances, may, by evacuating their Pores, or separating their parts, be render’d sufficiently opake; as Salts or wet Paper, or the Oculus Mundi Stone by being dried, Horn by being scraped, Glass by being reduced to Powder, or otherwise flawed…

Newton felt that opacity must arise from light getting scattered as it interacts with the gaps between the “least parts” of matter. This is not a general explanation of opacity, but it does work for oculus mundi, milk, and paper. Paper is made up of a bunch of transparent fibers woven together; light gets “trapped” and repelled by these fibers, making the paper look white. If you soak paper in oil, however, for instance if you eat a greasy piece of pizza on a paper place, you’ll find that the paper becomes translucent or even close to transparent.

Hopefully the transition to transparency for the oculus mundi stone is clear in the video; it was much easier to see it in person than it is to see it on the video. Anyway, this is another rather fun little science experiment that one can do at home. Hydrophane opal can be tracked down for sale online for quite cheap.