When scales tilt in favour of the fish

Evolution Researchers have found that certain fish have evolved a skin structure that keeps its reflectivity constant even when the angle or intensity of light varies. This helps them preserve their camouflage, writes S Ananthanarayanan

Reflecting surfaces have a way of throwing back less or more of the light that falls on them, as the angle of the incident light changes. An object may thus sometimes look darker than the background and become easy to spot. This could endanger living things that have adapted to merge and stay unnoticed by enemies and predators. T M Jordan, J C Partridge and N W Roberts at the University of Bristol have found that certain fish have evolved a skin structure that keeps its reflectivity unchanged even when the angle, intensity or other features of light are varying.

Nature of light

Light waves are like the waves that spread out on the surface of a pond or the waves that we see at the seaside. In the case of waves in water, the up and down movement of water causes adjoining water to move down or up, which similarly affects further adjoining water and so on. But while the movement of water is up and down, the movement of the wave is along the horizontal surface of the water or in the transverse direction. These directions are fixed because it is the weight of water that is causing the wave motion and gravity acts downwards. We could imagine that if gravity were to act not downwards, but towards the left, then the surface of water would not be horizontal, but vertical, and movement of water would be not up and down but right to left and back again. This would be like the water has been tipped and held up on its side and the waves that move forward along the surface would now be in the vertical plane.

Light waves arise not by the movement of any material thing, but by the variation of electric and magnetic fields. These fields are not affected by gravity and there is no associated concept of vertical or horizontal. The change in electric and magnetic fields can thus be in all possible planes, horizontal, vertical or turned at any angle, so long as they are transverse to the direction of the light wave.

And in general, light waves consist of field variations in all possible planes. But when light waves strike a plane surface, like a glass sheet, the surface is in a fixed plane and not in all possible planes. All portions of the light wave are thus not oriented in the same plane relative to the surface and the whole wave does not reflect or pass through in the same way.

Materials & light waves

There are materials that affect light waves in different ways, depending on the plane of vibration of the light wave. Thus, they may allow only light that vibrates in a particular plane to pass completely. In this case, light with a different plane would be weakened while light with the plane at right angles would be stopped altogether. The light that comes through would be vibrating all in the same plane, and this is called polarised light. Some materials allow all light to pass but affect the speed, depending on the plane of vibration. When a beam of light approaches a transparent surface, we can see that the planes of vibration could be either in the plane perpendicular to the surface or along the plane of the surface.

The portion that is along the plane of the surface would not be affected on reflection, but in the case of the plane that is perpendicular, the wave would not stay transverse when the beam changes direction, after reflection. At a particular angle, which depends on the angle at which the light wave enters and passes through the surface, this portion of the wave would be so turned around on reflection that it would not be reflected at all and it would completely pass through.

The light actually reflected, at this angle, would be the portion where the plane of vibration is polarised along the plane of the surface. Taking into account all parts of waves with vibrations along other directions, the amount of light reflected may be just half of what is normally reflected.This is the effect, of polarisation on reflection, which causes objects to become dimmer when the light comes off them at a particular angle.

Certain silvery fish like sardines and mackerel have evolved to appear the same colour as the scattered light that comes through the seawater around them. This makes them less visible against the background lighting, which helps them stay safe from predators. But if the light that reflects off their bodies was to dim at certain angles, then, they would still stand out against the background and come into view! Jordan, Partridge and Roberts report in the journal, Nature, that the scales of these fish have evolved a microstructure that takes care of dimming by polarisation on reflection.

Fish scales

The skin of fish consists of layers. Under the transparent outer scales is the silvery layer, or stratum argenteum, which is made up of many layers of two different, transparent materials. The first, guanine, has a strong effect on the direction of light that enters and the second, which is cytoplasm, has a lesser effect. Guanine has yet another property, its effect on light is different for different polarisations. The guanine layer thus continuously splits the light that enters into beams with opposite polarisation.

The Bristol group report that there are two kinds of guanine, one which splits light beams in one way and the other that splits them the other way! This structure of the fish skin thus contains many layers of transparent material that reflect or transmit light with different extent of deviation and again causing beams of different polarisation to diverge or converge.

The result is that even if light were to strike the exterior of the fish at an angle where the reflected light is strongly polarised, and hence weaker, the light that is transmitted through the outer layer is deviated and re-channeled so that it is soon again reflected just like at the outermost layer, and the extent of reflection is not materially affected by the degree of polarisation or the angle from which the illumination comes. Fish have thus evolved to counteract a natural phenomenon of polarisation of light and preserve their camouflage and security. Of interest to human researchers is the scale structure which helps them do it. Similar angle and frequency independent, multi-layer reflecting surfaces are used in applications like optical fibres, LED reflectors or channels for microwaves.

The construction of these devices has involved use of different materials to build the layers. The fish scale structure has many layers made of the same material, which provides advantages of uniform mechanical and heat conducting properties. Using the same design, polymers or a honeycomb that contains liquid crystal whose properties can be controlled electronically could lead to advances in optical devices used in industry.