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.
- http://news.discovery.com/animals/fish-break-law-of-physics-become-invisible-121021.html
- http://www.nature.com/nphoton/journal/vaop/ncurrent/full/nphoton.2012.260.html
