I attended the lecture at UCL yesterday, and was pleased to see a picture of Harry Potter on the screen. I’ve always been a fan of Harry Potter, and had come especially hoping to hear about how to make my own invisibility cloak.
Dr Akram Alomainy, from the School of Electronic Engineering & Computer Science, Queen Mary, University of London, told us about ways to make things invisible. The first way is conjuring, where stage magicians use illusions, such as mirrors and misdirection, to make it seem as though things simply disappear. Light travels in straight lines, which is the most basic bit of information about waves, except obstacles reflect, diffract and scatter waves.
Another way of making invisible could be to bounce the waves away by diffraction. This would be slightly difficult, since white light is made up of all the rainbow colours added together, and the different coloured lights travel at different speeds. Changing your refractive index would make you invisible, as the angle the light bends at depends on the material, so if you have the same refractive index as the air around you, light will pass through you like the air around you, so there would be no refraction effect. However, you would have to change every single part of your body to have the same refractive index, which would be so complicated. Water beads, which Dr Alomainy apparently purchased for a mere 99 pence, have the same refractive index as water. If you leave them immersed in water for 6-8 hours, they soak up the water, and become invisible to our eyes when dropped into water. This short live demonstration after the video induced a ripple of excitement around the auditorium.
Even if a cloak was invisible, you would have to be transparent too for it to function. The hair of the polar bear is translucent, so it appears to be almost white in colour, even though its skin is actually black. Octopuses are able to make themselves invisible, as they have a highly reflective skin layer, and in the video we were shown, it became the same colour and shape as a plant it decided to rest on. This lead Dr Alomainy on to the art of camouflage. Optical camouflage bends light around the wearer to make it appear like the wearer isn’t there, while special material reflects light exactly. BAE Systems has developed an active camouflage technology called the “ADAPTIV” system, which has been used by the military to protect tanks and other military vehicles from detection. Bending light would make something invisible, which is like what happens in a mirage. Lights rays are bent due to the layer of hot air above the ground, and an upper layer of cool air, which produces a displaced image of distant objects.
A cloak of the future would require a need to engineer the materials at the scale of the wavelength of light, using nano-technology and metamaterials. There has been a real cloak that has been produced, but it only works for certain lights. Real invisibility, at the moment, has only been generated in the lab, and when the mechanisms of real invisibility have been worked out, ethical issues will be raised. Like J.K. Rowling said (although about Tom Riddle’s diary in Harry Potter and the Chamber of Secrets and not about Harry’s cloak), “A suspicious object like that, it was clearly full of Dark Magic.”