Revealing your secret superpower – How you can see polarized light by looking for “Haidinger’s brush”

With the International Year of Light now in full swing, there’s been a lot of talk on this blog about how light is useful for everything from medicine and the arts to technology and astronomy. But what I want to tell you about is an astonishing – and largely unknown – light-based superpower that you perhaps don’t even realize that you have. It may sound bizarre, but using the naked eye – and with no additional gadgets whatsoever – you can detect whether or not light is “polarized”. And in the video below, my colleague Louise Mayor, who’s features editor of Physics World magazine, shows you how.

If you were wondering what polarized light is, remember that light itself is nothing more than electric and magnetic fields moving up and down along axes at 90 degrees to each other. The light we see from natural sources such as the Sun is often “unpolarized”, which means that the electric fields of the waves making up the light are orientated in many different directions. Polarized light simply means that the electric fields of all the waves are aligned and oscillate on the same axis.

Many animals can see polarized light – birds and bees, for example, use it to navigate as they fly. But how can you see this kind of light? Well, first you need some polarized light. One option is to look at ordinary, unpolarized light while wearing, say, a pair of polarized sunglasses. Another option is to use a liquid-crystal display (LCD) computer screen. As Louise explains, your best is to set the screen to display plain white or blue – in fact, in the video we’ve included a frame of a few seconds displaying just a single colour. So if you click on “full screen” and pause the video, you’ll have an instant source of monochrome, polarized light.

All you then need to do is to stare at the polarized light. It might take a bit of practice, but you should eventually become aware of an image at the very centre of your vision. It’ll look a bit like a small yellow bow tie crossed with a blue bow tie 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 work out which axis the light’s polarized along. Seeing Haidinger’s brush takes a bit of patience: if you’re looking at an LCD display, you can check if you’re doing things properly by tilting your head slowly from side to side – the brush should not change orientation.

You can find out more about this weird effect in a fascinating article in the March issue of Physics World magazine, published by the Institute of Physics. It’s written by David Shane from Lansing Community College in Michigan. “Unveiling your secret superpower”, where he loves to tell his students about Haidinger’s brush. Shane admits it’s a subtle effect that most of his students can’t see at first. But it’s worth persevering, he says, because if you can see the brush, then you can look for polarized light anywhere. Shane suggests, for example, looking at light that’s reflected off a still lake. “Don’t give up if you can’t see it at first!” he insists.

I’ll finish by pointing out that Haidinger’s brush is an “entoptic” phenomenon, which means that the image is created in the eyeball itself and so you’ll never see a photograph of it. As for what causes Haidinger’s brush…well that’s still a mystery. So while we try and figure that the little puzzle out, I suggest you just try and have a go yourself while enjoying what’s a fun and fascinating piece of physics.

* Can you see Haidinger’s brush? #SecretSuperpower


Matin_DurraniMatin Durrani is Editor of Physics World magazine, which is published by the Institute of Physics.

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