Monday, 30 April 2012

Milly: Polarization Paradise


I've not been the most active of writers on this blog of late but, fear not, I'm going to write another series of posts as I blog/blather from the field.

Lizard Island, Australia. Photo: Michael Bok.
In just under a month from now, my lab and I (Ecology of Vision Group) will be flying to Australia, Lizard Island, on a mission to unveil more secrets about the vision of marine animals. You may be wondering why it is necessary to travel across the world to do this. Well, aside from the fact that scientific success increases significantly when in an idyllic location (obviously), we need access to Australia's diverse range of reef dwelling beasties, including the charming octopus and the not so charming mantis shrimp, more likely to rip your hand off than to shake it.

A mantis shrimp (stomatopod).
Photo: web.
Our team have collected all of the gear we will be needing for experiments: LCD screens, perspex tubes, lightbulbs, cameras, 3D glasses and milk. Now, it may sound like we are planning to watch a film, but actually we are going to do some serious and exciting science.

The word that binds our research together is polarization. If my colleagues and I were the mince, polarization would be the egg that binds us together forming the burger (?!) that is our group. Slightly off the beaten (egg) track.

Serious science time:


What is polarization?
Unpolarized light coming from a light source is oscillating at all
possible angles in that plane, however, when it is passed through a
filter (polaroid) it becomes polarized, oscillating only at one angle.
When applied to light, polarization means the direction that the light is oscillating in. If you imagine that you are holding a rope and you shake it up and down, waves form, travelling down its length. You can shake the rope from side to side, or also swirl it round forming a rotating pattern that also travels along the rope. This same idea can be applied to light as it too oscillates as it is travelling along as a wave. Just like the wavelength of light can inform an animal of the colour of something it can see, polarization can also provide additional information as light bounces off different structures or is scattered by particles.

How can an animal detect polarized light? 
We, as humans, know that polarized light exists around us, but unfortunately, without polaroid filters, we cannot see it. Unless of course you are one of the lucky few who have deliberately tried to view strong sources of polarized light such as LCD monitor outputs and are now cursed, forever having a strange yellow bow tie shape appear randomly on the desktop. It's called Haidinger's brush if you fancy having a go yourself. To detect polarized light oscillating at one angle, your photoreceptors must be aligned at that same angle, to absorb the maximum amount of light. If your photoreceptor is, say, 90degrees out compared to the polarized light, then it's not going to absorb very efficiently. This sort of arrangement of photoreceptors where one lies at one angle and a second, connected photoreceptor is lined up perpendicular to it, is very common in invertebrates and is the basis for their polarization vision. Simply put, it allows them to compare the outputs of these two receptors and figure out what angle the light is oscillating at.

Why is polarization vision useful?
Unpolarized light bouncing off the surface of the
water becoming polarized horizontally.
Photo: Wehner (2001).
At first it might sound like polarization vision could be disadvantageous, since you have the potential to lose information every time polarized light hits your receptors at the wrong angle. What it does do, however, is convey valuable information. When light bounces off a shiny surface, such as water, much of the reflected light becomes horizontally polarized (oscillating at the same angle as the water's surface). If the light hits the water at Brewster's Angle, then all of the light is horizontally polarized. Now, imagine that you are a water-seeking insect where the survival of your species depends on you reaching water to mate and lay your eggs. Some strong selection pressures there. If you have receptors aligned horizontally and pointing down towards the ground, you have a perfect water detecting device. This is a common feature of water-seeking insects. Unfortunately, lots of man-made surfaces are shiny so if you have ever wondered why you find dead beetles and mayflies on the highly reflective bonnet of your car...now you know. Polarization vision isn't just useful for this one task, light is also polarized as it travels through scattering media such as water, or the atmosphere. As the light scatters it becomes polarized at an angle depending on the incident light. If this is happening millions of times in the sky as the light travels towards the Earth, a predictable pattern is formed which acts as a map to navigation and orientation in bees, beetles and other insects where the landscape is complex, moving and changing or devoid of any useful visual landmarks on the ground. The same applies underwater.


Invertebrates such as insects, crabs and cuttlefish have polarization
sensitive cells in the eye consisting of perpendicularly oriented
light absorbing microvilli. You can see the two orientations in the
TEM image of dragonfly photoreceptors above.
Photo: Meyer and Labhart (1993)

I still haven't got to the bit where I explain what we are doing in Australia. I think that is quite enough for one post, time for a cup of tea.





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