Staying well out of sight
(appeared in Nov2016)

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Being transparent is sometimes not sufficient to be invisible, says S.Ananthanarayanan.

Taking on the colours of the surroundings is a good way to avoid detection. However, this may not work when one is in featureless surroundings. In such a case, what is needed is to be transparent. Even so, being transparent helps mainly when one is lit from behind and not quite as well when lit from the front. This is because there is almost always a bit of light that transparent surfaces reflect, which would give the transparent object away.

Laura E. Bagge, Karen J. Osborn and Sönke Johnsen from Duke University, North Carolina and the Smithsonian National Museum of Natural History, Washington DC, report in the journal, Current Biology, the adaptation by a class of sea water dwelling organisms to minimize reflection of light by their body surface, as a means to avoid detection.

The animals studied are a group calledhyperiids, which inhabit the upper column of sea water, short of deep water, and are covered by a light shell. The organisms are generally transparent. However, as the downward illumination at the depths where the animal is found is much greater than horizontal illumination, even a small extent of reflection by the animals would make them stand out when viewed horizontally. The animals have thus been under evolutionary pressure to minimize the level of reflection by their surface.

The reason that transparent surfaces also reflect part of the light that falls on them is that there is a change in the speed of light when it passes from the surrounding to the transparent material and again when light passes out, to the surroundings. The speed of light in a material depends on the way the material supports electric and magnetic variations that light consists of. The abrupt change in these properties when light passes from one material to another results in part of the energy not being transferred into the material but sent away as a reflection. One way of reducing the reflection from a surface would hence be to create conditions so that the change in the speed of light is not abrupt, but a little smoother.

The Current Biology authors report that they used a scanning electron microscope to investigate the skin surface of seven varieties of hyperiids and discovered features, which have not been documented so far, which may be the way reflectance is reduced from these surfaces. The researchers found that the skin on the legs of one form of hyperiid is covered by nano-protuberances some 200 nm in height and the skin of this and the other hyperiids also had a layer of spheres, in the same range of dimensions.

The paper notes that the effect of such protuberances of dimensions less than the wavelength of light was first observed in the case of corneas of moths and some butterflies. Moths usually come out at night and to be able to see, they need to let in as much of the light that comes to their eyes as possible. The cornea, or the outer layer of their eyes, which has thus evolved to minimize reflection, and maximize transmission, also has protuberances of sub-wavelength dimensions. The minimized reflection also serves to make the moth more difficult to detect, in the dark, which protects it from predators. It was also later found that the wings of the moth and cicadas have a microstructure so that the area of chitin (hard tissue) the light strikes increases as one gets nearer the surface, the paper says.

A gradually changing structure, at a size less than the wavelength of light results in gradual change in the speed of light waves and hence greater proportion of light being transmitted, as opposed to reflected. The paper mentions that the theoretical reduction in reflectivity, based on the dimensions of the surface structure, is verified in practice and experiment. The researchers found that the legs, which account for most of the surface area of one variety of hyperiids, were almost fully covered with an ordered, periodic array of nano-protuberances, 200 nm high and spaced 96 nm apart and conical in shape, just like what is seen on the transparent wings of insects and corneas of moths.

The paper notes that a thin transparent layer where the speed of light is in between the speed on the two sides of a surface, could be arranged, depending on the wavelength of the light that was falling, to completely eliminate reflection. This property has been made use of to cover lenses used in the laboratory or industry, to reduce reflection at the surface. While instances of this device have not been seen in nature, the paper says, most of the body skin of the hyperiids were covered with a dense monolayer of identical spheres, which appeared to be layer of a form of bacteria. X Ray analysis showed that the material was all organic and some of the spheres, which were not part of the skin but external attachments, also seemed to be undergoing division. These spherical bodies measured from 52 to 320 nm, which would affect reflectivity and appeared to be acquired from the environment.

Further analysis showed that the nanostructure clearly reduced reflectivity of the surface, particularly when light fell at glancing angles and was more likely to be reflected. A clean, smooth skin surface was found to have reflectivity of 0.6 to 1% when light fell squarely on the surface, increasing to 7% to 75% at glancing angles. With the nanostructure, however, reflectivity was less than 0.5% for a broad range of wavelengths and angles of incidence.

Helper bacteria

Another instance where bacteria help organisms stay unseen is in the case of the Hawaiian Bobtail squid or the Flashlight Squid. While foraging at night in the shallow Pacific Ocean (and some parts of the Indian Ocean), the squid presents a silhouette or shadow against the moonlight or starlight, to predators below, and becomes a sitting duck target. The squid has evolved to support a colony of light emitting bacteria, called Vibrio Fischeri, which live off the sugar and amino acid nutrients from the squid, and repay by getting luminescent, as and when required, for the host to hide its shadow.

Increasing reflectivity

A converse of the strategy described so far is employed by some silvery fish, like sardines, carp and mackerel, to stay out of sight in well lighted waters. Although their scales reflect diffuse light, to match the background, there is a property in reflection which makes the light a little dimmer, on reflection. It has to do with light being electrical and magnetic vibrations, in all planes transverse to the direction of motion of the light. On reflection, certain planes are reflected more easily than others, leading to a drop in intensity. These fish have a multilayered and transparent microstructure in their scales so that transmitted light is channeled to reflect and compensate for the loss. The light coming off the fish is hence of the same intensity as the light that fell on it and that is there in the surroundings, and the fish stays out of sight!


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