The Royal Society

Seeing the invisible

Posted by on 13 July 2011

By Annabel Slater, digital volunteer at the Royal Society Summer Science Exhibition

Eeyore’s mournful expression peeks out through a loose cradle of optic fibres. The little donkey sits in the middle of the translucent bundle, acting his incredible part in the Royal Society Summer Science Exhibition. For in this exhibit, he is no mere toy donkey. He is the fantastic, light bending, Invisible Eeyore.

Ulf Leonhardt, Professor of Theoretical Physics at St. Andrews University, explains that the Eeyore display is mimicking the powers of the Invisible Woman, who can bend light around herself so that she vanishes from sight. The Invisible Woman’s forcefield supposedly sends light straight back at the viewer, without any being absorbed and reflected as colour.

Ulf shines a light through one end of the optic fibre bundle, showing how the light travels straight through the fibres and curves around Eeyore to appear at the other end. He blocks some of the light using a plastic sheet printed with numbers, and the black numbers show visibly at the other end. The display only represents the bending of light from one direction, but nonetheless, it is fascinating. “If you could do this from all directions, you could make [Eeyore] disappear.”

Ulf is part of a group of scientists presenting a light-bending exhibit entitled ‘Geometry and light: the science of invisibility’. The exhibit is a perfect combination of superheroes, science-fiction, mysterious displays, and friendly scientists.

Moving to another part of the exhibit, Ulf shows me how objects can be made to vanish inside a clear medium, like a liquid. He holds a cube of clear jellyish material above the surface of a tank of water, then lets it drop. The cube seems to vanish as soon as it touches the water, and he invites me to swirl through another seemingly empty tank with a fishing net. I dunk it in and scoop up a huge pile of clear, squishy marbles. It’s immensely satisfying, like pond-dipping for magic frogspawn, and I feel the urge to reach in and grope around for an invisible handful of the squeezy shapes. Luckily for me, this sort of messy visitor interaction is perfectly acceptable, and people can even take home their own free vial of invisible sphere and water.

Image taken by Georgia Lockwood Estrin, digital volunteer at the Royal Society Summer Science Exhibition

I ask if the shape of the object causes them to vanish in water.  No – the objects are made out of a material that matches the water’s refractive index, which is the amount that light is bent when passing through an object. Light passing through the water also passes through the spheres and cubes without being scattered or reflected. If the objects were dropped into another liquid of a different refractive index, then I would be able to see them.

Further along, I see a low table spattered with messy pieces of clear jelly shapes. They have an alluring and eerie blue glow in the exhibit’s lighting, and I have to investigate. Another scientist, Joanna, shows me how the jelly shapes – made from simple kitchen jelly – refract and bend a ray of light shot from a laser pointer, sending the ray zigzagging across a patch of table.

I can’t help it. Joanna’s wielding a glowing laser pointer like a wand, and the whole team are wearing academic gowns that look like splendid black robes. They’re bending rays of coloured light and casually vanishing solid shapes in liquids. Never mind the Harry Potter premiere, right now these scientists are the closest things to true wizards in the whole of London, and I have to ask about invisibility cloaks.

“I think we’re a long way from anything like a Harry Potter-style Invisibility Cloak,” Joanna says with a smile. She shows me how the jelly bends red light differently to green light, because of their different wavelengths. This is why white light passing through a glass prism spreads out into separate colours. A true invisibility cloak would need to refract all the different wavelengths of light in the same way.

Still, this exhibit shows the possibilities of bending light, and explains the science behind the fibre optic technology revolution of the past few decades. The more we learn about how to manipulate light, the more we can use it – and I can’t wait to see – or, well, try to see – where we will.

Here is a video of Professor Ulf Leonhardt talking about his exhibit Invisibility science