The Royal Society

2011 Summer Science Exhibition 

Seeing the Invisible

"She’s wielding a glowing laser pointer like a wand, and the whole team are wearing academic gowns that look like splendid flapping black robes.  They’re bending rays of coloured light and casually vanishing solid shapes in liquids.  Forget today's premiere of Harry Potter.  Right now these people are the closest things to true wizards in the whole of London."

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 2011 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.  Her forcefield sends the light straight back at the viewer, without any being absorbed and reflected as colour.

“Why Eeyore?” I ask. 

He smiles and shrugs.  “Well, we just needed any toy.”

But I think it is a very appropriate choice.  Gloomy Eeyore probably would want to hide from the world, if he could.

The Professor 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 him disappear,” says Professor Leonhardt.

He is part of a group of scientists hosting a light­bending exhibit entitled ‘Geometry and Light: The Science of Invisibility’.  The stall is a perfect combination of superheroes, science­-fiction, mysterious displays, and friendly scientists.

Moving to another part of the exhibit, Professor Leonhardt 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
pond­dipping for magic frogspawn, and I want to reach in with my hand 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.

I ask if it is the shape of the object which causes them to vanish in water.

Ulf explains it's not the shape that matters, but the substance.  The objects are made out of a material 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.

This is the science behind the Invisible Man­ a man who could change his own body cells to match the refractive index of air.

Further along the stall, I see a low table spattered with messy pieces of clear jelly shapes.  They have an alluring and eerie blue glow in the stall’s lighting, and I have to investigate.  Another scientist, Joanna, shows me how the shapes, which are­ made from simple kitchen jelly­, refract and
bend a ray of light shot from a laser pointer.  The ray is sent on a journey of zigzagging angles across the table.

I can’t help it.  She’s wielding a glowing laser pointer like a wand, and the whole team are wearing academic gowns that look like splendid flapping black robes.  They’re bending rays of coloured light and casually vanishing solid shapes in liquids.  Forget today's premiere of Harry Potter.  Right now these people are the closest things to true wizards in the whole of London!  I have to ask about invisibility cloaks.

“I think we’re a long way from anything like a Harry Potter­style Invisibility Cloak,” Joanna says with a sympathetic 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 stall 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.  We'll have to wait and see- or try to see-­ what the future holds.

[Originally available at http://blogs.royalsociety.org/science­live/2011/07/13/seeing­the­invisible/]