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Optical physics

Unveiling your secret superpower

02 Mar 2015
Taken from the March 2015 issue of Physics World

If you thought weird visual abilities belonged to the realm of the few, think again. David Shane describes a bizarre talent that most sighted human beings have but probably don’t know about

We often marvel at the special abilities of animals, such as eagles being able to see four to five times farther than the average human, or how cats always seem to land on their feet. But take a look at our own species and you’ll find we have an astonishing and largely unknown natural talent of our own. It is a bizarre skill and sounds like science fiction, but with the naked human eye – with no additional gadgets whatsoever – most of us can detect whether or not light is polarized, and can even determine the light’s axis of polarization.

Before we begin, remember that light behaves as a transverse electromagnetic wave; it consists of electric and magnetic fields oscillating on perpendicular axes. Light that we observe from natural sources such as the Sun is often “unpolarized” or “randomly polarized” – two descriptors that both mean the light waves reaching us have electric fields orientated in many different ways. Polarized light simply means that the electric fields of all the waves are aligned and oscillate on the same axis.

As a physics teacher, our ability to detect this polarization is something I love sharing with my students. The first step in doing so is to find some polarized light to look at. In class what I normally do is get my students to look at an ordinary non-polarized light source through dedicated Polaroid films. If you have a pair of polarized sunglasses, those should also work well. Another handy source is an LCD computer monitor; for the best result, set the screen to be plain white or blue, which you can do in most text- and image-editing programs.

Now for the fun part. Stare at your polarized light, and – though it may take practice – you should become aware of an image at the very centre of your vision. The image looks a bit like a small yellow bow tie crossed with a blue bow tie (figure 1) and is known as Haidinger’s brush, after the Austrian scientist Wilhelm Haidinger who first reported it in 1844. The blue bow tie is aligned with the electric field of the light you are observing, and so you can use this to determine the axis of polarization of the light. In case you’re wondering how big this image is, for me it’s about the width of my thumbnail held at arm’s length, or approximately two words wide if you’re reading this magazine from a typical distance right now.

Seeing Haidinger’s brush takes a bit of practice, but an encouraging check you can use if you’re looking through a polarizer of some kind is to rotate it, which should make the brush rotate as well, since it’s aligned with the polarization. If you’re looking at an LCD display, on the other hand, tilt your head slowly from side to side and the brush shouldn’t change orientation. It’s a subtle effect, and most of my students report that they see nothing…at first. You might also find that one of the bow ties is more apparent to you than the other – I can see the yellow more readily than the blue myself, but I’ve had students report the opposite.

Incidentally, I find it interesting to teach my students about Haidinger’s brush because if they tell me they can’t see it, I really cannot get behind their eyes and help them. It is an “entoptic” phenomenon – the image is created in the eyeball itself – which is why you will never see a photograph of it.

If you can see the brush, then you can look for polarized light anywhere, not just when looking through your sunglasses. Try looking at the sky, away from the Sun; the sunlight that reaches you via scattering in the sky is partially polarized – an effect that is most pronounced at 90° to the Sun. You can also try looking at reflections, such as in a still lake. Don’t give up if you can’t see it at first!

Unsolved mystery

At this point you are probably wondering: what causes Haidinger’s brush? Well, as it involves the human eye, we would expect the answer to involve at least as much biology as physics. But while a host of explanations have been offered over the last century, nobody really knows for sure. (This is something else I like to tell students – that the world is still filled with common phenomena for which our explanation is missing or uncertain. Fear not, therefore – there is still much work to do!)

One recent explanation was offered by a team led by the physicist Albert Le Floch at the University of Rennes in France, which explains how this phenomenon might occur based on the geometry and biology of the eye (2010 Vision Research 50 2048). The human eye contains two types of photoreceptor: rods and cones. The cones are sensitive to colour – and, well, Haidinger’s brush is coloured, so perhaps here we will find our explanation? Cones come in three varieties labelled blue, green and red, corresponding to the frequencies of light to which they are most sensitive.

The explanation of Le Floch et al. depends on just a few critical facts. First, the blue cones, which are the most rare, are missing from the centre of the fovea – the area at the middle of the retina behind the pupil – but away from that centre they are found scattered in among the red and green cones in a circular geometry. Second, when light moves from one material to another, the amount of light transmitted or reflected depends on the angle of the light’s polarization relative to the surface it is incident upon. What this means is that if polarized light enters the eye and then strikes the off-centre blue cones – at an angle slightly off the normal – more light will be transmitted into blue cones along the light’s polarization axis than those along a perpendicular axis. And voilà – we have a vision response that depends upon polarization. Or so says this theory at least.

What good is this to you? Well, one use of the phenomenon is to help correct “lazy eye”, since the brush always appears in the centre of the vision. But for most of us, as we celebrate the International Year of Light, it’s just a fun and fascinating piece of physics to notice and share.

  • In this video, features editor Louise Mayor explains how to see Haidinger’s brush

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