Invisibility Tricks

By Alice Orszulok

Long before Harry Potter acquired his invisibility cloak, scientists have been infatuated with the idea of inventing something that can hide objects .

In Harry Potter’s first year at Hogwarts, he is sent a special cloak that gives him the power of invisibility when worn. This cloak allows him to get up to all kinds of mischief, such as staying out after dark and going to certain areas of the castle where he’s forbidden to be. A real-life invisibility cloak would allow us to do much more than just sneak around school undetected (although this would be pretty cool) and could be used in the military, microscopy, biomedical sensing and energy harvesting devices.

There have been two recent proposals for different invisibility devices: the first works by concealing objects from different light frequencies, and the second uses antennas to make objects invisible to radars, although humans can still see it.

To understand how invisibility works, you first need to understand how light affects the way we see. Light works like waves. Out of the whole electromagnetic spectrum, we can only see light within a certain range of wavelengths and frequency, which we call visible light (other animals can see different wavelengths than we do). When we look at an object, we are looking at the light that is not absorbed by that object. For instance, green plants are green because they absorb all of the colours of the visible spectrum except the colour green (green wavelengths), and your mirror reflects all of the colours of visible light.

Current invisibility cloaks work by redirecting types of radiation so that they bend around an object to make it invisible. However, this can only work at specific light frequencies at any one time, and can make other colours even more visible. For instance, if you make an object invisible to red light and it was illuminated by white light (which contains all the colours) the object would turn bright blue!

Scientists at the University of Texas have designed an electrically powered invisibility device that can hide objects over a wide range of light frequencies. “If you want to make an object transparent at all angles and over broad bandwidths, this is a good solution,” professor Andrea Alù, lead author on the study told BBC. “We are looking into realising this technology at the moment, but we are still at the early stages.”

However, the scientists admit that creating a fully functioning invisibility cloak is impossible. “If you suppress scattering n one range, you need to pay the price, with interest, in some other range,” professor Alù said.

This is why the second design is handy. Although it doesn’t make objects invisible to humans, by using small antennas that radiate radio waves that cancel out waves bouncing off the object, it makes the object invisible to radar. Radar (radio detection and ranging) is a system used to find objects, such as airplanes and submarines. This technology would therefore help hide military vehicles, but could also help in other ways, such as by cloaking structures that block mobile phone signals from passing through.

Although both designs don’t come anywhere close to Harry Potter’s cloak, scientists believe that invisibility is increasingly possible.

Sources [BBC and Science Daily]

Key term [Electromagnetic Spectrum]

The electromagnetic spectrum is a range of all electromagnetic waves organised according to their frequency and wavelength. This includes gamma rays, microwaves, radio waves and visible light.