Ever fancied having a superpower? Something you can call upon when you need it, to hand you extra information about the world? OK, it’s not X-ray vision, but your eyes do have abilities that you might not be aware of.
We are all familiar with colour and brightness, but there is a third property of light – the “polarisation” that tells us the orientation in which light waves are oscillating. Animals, like bees and ants, use the polarisation patterns in the sky as a navigation aid. But few people, even in the scientific community, are aware that humans can perceive the polarisation of light with the naked eye.
In research we’ve just published in Proceedings of the Royal Society B, we used an experiment that was originally designed to test the visual abilities of octopuses and cuttlefish to investigate our human ability to perceive this polarised light.
We already use polarised light
Imagine a skipping rope is a light wave travelling through space. If you move the rope from side to side, the wave you make is horizontally polarised. But if you shake it up and down you create a vertically polarised wave. Generally, light is a mixture of polarisations, but sometimes – for example in parts of the sky, on your computer screen and in reflections from water or glass – a large percentage of the waves are oscillating in the same orientation. This light is described as being strongly polarised.
You will probably have come across technology that is built around polarised light before. For example, “Polaroid” sunglasses work by blocking out polarised light which is reflected from shiny surfaces such as car bonnets or the surface of water. This is possible because light reflected into our eyes from horizontal surfaces is horizontally polarised and the sunglasses have a structure like a picket fence, so they only let in vertically polarised oscillations, blocking out the horizontally polarised bright reflections. Polarised light is at the heart of modern 3D cinema and LCD computer screens, smart phones and tablets.
So if polarised light is actually pretty common outdoors, in your home and in your office – how come you didn’t notice anything special before now?
Humans perceive polarised light using “Haidinger’s brushes”, a subtle visual effect which appears like a yellow bow-tie at right angles to the polarisation angle. You may also see a bluish bow-tie at right angles to the yellow one. The effect originates within the eye itself and is not an image of a real external object, so Haidinger’s brushes usually fade in a couple of seconds as your brain processes them out. This is one of the reasons that few people notice them day-to-day, and why they have previously been fairly difficult to study.
By using LCD screens capable of constantly refreshing the effect, we were able to make the first measurements of the dynamics of Haidinger’s brushes, confirming the prediction that some individuals would perceive the orientation of the bow-tie to “flip-flop” as the polarisation angle is rotated.
Our results shows that your cornea can dramatically affect how you perceive polarised light. As the optical properties of the cornea vary between individuals, this may also explain why people often report their experience of seeing Haidinger’s brushes differently.
To see Haidinger’s brushes for yourself, look at a blank white portion of an LCD screen on a computer, tablet or phone. Tilt your head from side to side and faint yellow and blue bow-ties, slightly larger than your thumb, should become visible. With practice, you can then see them in the blue parts of the sky at 90 degrees from the sun, particularly at sunrise and sunset.
Skylight polarisation patterns, caused by light scattering in the atmosphere, are such that the long axis of the yellow bow-tie will point approximately towards the sun.
What’s going on in the brain
In previous studies LCD screens have been used to test polarisation sensitivity in aquatic organisms. Our study tested the limits of human polarisation sensitivity, developing special filters to vary the percentage of polarised light reaching the eye from 0% to 100%.
This was to establish the minimum percentage polarisation at which Haidinger’s brushes could be detected. Among 24 people, the average polarisation sensitivity threshold was 56%. Some people could still see Haidinger’s brushes when the light was less than 25% polarised – not quite as good as cuttlefish but still better than any other vertebrates tested to date.
The ability to see Haidinger’s brushes is caused by circularly symmetric organisation of carotenoid pigments in the macula (an area that covers and protects the central part of the retina). Blue light which is oscillating parallel to these pigment molecules is strongly absorbed. White light, which is depleted in blue, appears yellow, which explains the yellow bow-tie effect. The blue parts of the brushes are thought to be generated by the brain in response to the unexpected presence of yellow.
As AMD is currently the leading cause of blindness in the developed world and finding an early stage diagnostic indicator of this before any actual sight loss is incurred is a research priority. It is our hope that polarisation sensitivity could be used to investigate and ultimately monitor any changes in the organisation of the pigments happening in the early stages of this degenerative eye condition. More work is needed to assess the medical potential of these kinds of tests.
Haidinger’s brushes also provide a demonstration of the physics of light and the anatomy of the human eye. By taking the polaroid layer off an old LCD screen you can make your own simplified version of our test; black and white letters turn into contrasting polarisation angles once the polarising film is removed.
At a recent science festival I tried to get people to take an “octopus eye test” by reading the hidden letters using their polarisation sensitivity alone. It went down a storm, except with one little boy, who was terrified of the accompanying octopus headdress. Time to work on a less intimidating super-costume.