a new study from the University of Bristol has shown that fish such as sardines, herring and sprat have evolved a way around this, which has helps them to avoid predators.One of the basic properties of physics is that when light is reflected from a surface it becomes polarized, but
Photons, which are the basic 'particles' of light, vibrate as they move about. The direction they vibrate in tends to be random from photon to photon, unless something acts on them to make their vibrations not-random. For instance, when the Sun is shining down on you from directly above, the light from it shows an almost entirely random assortment of photon vibrations. This is called 'unpolarized' light. However, at the same time, light coming at you from any direction other than straight up shows an increasing amount of 'polarization' — where the direction of the photon vibrations is more similar — with the maximum polarization being from the horizon.
Polarization happens when the photons interact with something else, either by being absorbed and re-emitted, or by being reflected from a surface. A filter can also polarize light, by only allowing photons that vibrate in a specific direction to pass through — this is how polarizing sunglasses work. However the polarization happens, it generally causes a decrease in 'reflectivity' — the amount of light that is seen after reflection.
These species of 'shiny' fish — sardines, herring and sprat — have tiny crystals of guanine in their skin that are highly reflective, and it was assumed that the fishes' skin would polarize light as any other reflective surface does, which would cause a drop in the amount of reflectivity. However, PhD candidate Tom Jordan and his supervisors found that light reflected from these fish is unpolarized, and that the reflectivity of the light stays high.
The fish accomplish this feat of light by having a mix of two types of guanine crystals in their skin. Each type has different optical characteristics and thus polarizes the reflected light differently. So, even though each individual guanine crystal in the fishes' skin does polarize the light it reflects, the overall effect of the mix of crystals through the layers of skin results in reflected light that is unpolarized.
"We believe these species of fish have evolved this particular multilayer structure to help conceal them from predators, such as dolphin and tuna. These fish have found a way to maximize their reflectivity over all angles they are viewed from. This helps the fish best match the light environment of the open ocean, making them less likely to be seen." said Dr Roberts, according to Science Daily.
Jordan believes that this discovery may inspire new designs for optical devices.
"Many modern day optical devices such as LED lights and low-loss optical fibres use these non-polarizing types of reflectors to improve efficiency. However, these human-made reflectors currently require the use of materials with specific optical properties that are not always ideal. The mechanism that has evolved in fish overcomes this current design limitation and provides a new way to manufacture these non-polarizing reflectors."
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