Cloaking Devices

Cloaking devices are a big thing in Sci-Fi stories. They are the dream of some physicists working hard to bend light round objects. So far some success with small objects has been won, but we’re still a long way from something as big as the Starship Enterprise.

Nature doesn’t hang around waiting. The evolutionary pressure of predation is a great spur and small fish have had a lot of predatory pressure to evolve for quite a while. Jordan, Partridge and Roberts from Bristol U have been taking a look at fish like sardines and herrings, or at least trying to when the light is right (1, 2).

They have shown that the silvery skin, which gives that shimmery appearance as they move in and out of sight, has layers of crystals in them. Each layer has the crystal axis perpendicularly oriented to the next layer. The crystals are guanine.

The article in Discovery News focuses our attention on guano as though it was something that we higher beings might find just a tad distasteful (1). But we should remember that we are laced through and through with guanine as it is one of the four components of our DNA. Without guanine we would just be part of an very old primordial soup.

So having established that guanine is good for us, we can continue eating sardines and herrings without thinking about bird droppings. The way their skin works to confuse is by not confusing. When light is reflected from a mirrored surface, the fraction of the light that is reflected is the fraction that is polarized horizontally – that is parallel to the surface. The vertical components do their best to penetrate.

Our multilayered little piscatorial wonders have alternate layer which tackle the horizontal and vertical components because of their crystal orientation. Hence, wonder of wonders, they don’t reflect and if they don’t reflect, the predators don’t see them. So they, in their way, are like little stealth aircraft which don’t reflect radar waves back to the detectors, and “fly” about in the sea while being difficult to make out clearly.


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