Where do mirages come from? What mirages and nanotubes have in common? Could we become invisible on desert? Or maybe nanotechnology has another clue how to make us an invisibility cloak? Let’s see!

How light travels?

We’ve already answered this question in part 1 but to remind: light goes ahead super-fast and always finds the fastest way possible between two points (this is the Fermat principle). The light for instance goes through glass 1,6 times slower than through vacuum (where even air isn’t present). The quantity which reflects how slowly light travels is a refraction index. Therefore the index equals 1,6 in glass (and 1 in vacuum). In the previous post we made metamaterials with any given refraction index, so the light travelling through it could bend it’s way around the cloaked object. Today however our task will be to make a cloak which will bend the light away even before making a contact with it!

So where is that oasis?

Under no circumstances should you believe anyone or anything on desert. Your eyes in particular. Let’s think about mirage. A mirage is an illusion when we see an oasis or a city in places where they don’t exist. They are not hallucinations though! A mirage is a physical phenomenon and objects we see truly exist but miles or dozens of miles further.

How it works? Let’s see in Fig. 1. When the sand is heated by the sun, the air just above the ground Is also heated. The air further from the ground is cooler. The hot air is more thin and the cool air is more thick. It turns out that the light prefers thin air as it goes faster through it than through thick air (refraction index is smaller for thin than thick air). Let’s imagine this: A travel through hot, thin air is like swimming in water and a travel through cool, thick air is like swimming in honey. On the right side in the picture there is an object and on the left side there is a man. If the air was of the same density everywhere, then the light should travel through shortest way possible – a straight way (the top dotted line). However the air has different temperature on different height and because air is hotter (and thinner) on lower height, the light bends it’s way downwards.

The light bends it’s way. So what? Let’s compare this effect to a popular school joke: It’s you and your friend and lots of people everywhere. You are on the left and your friend is on your right. You both are looking in the same direction. You’re discretely bending your arm around your friend’s back just to touch the friend’s shoulder on the other (right) side. Your friend involuntarily turns right to see who has just poked him while you just can’t hide your laughter. Congratulations. You’ve just deceived your friend by bending your arm and attacking him from another direction! Similarly the light is bending it’s way and “poking” our eyes from another direction, so we see the object in different place from where it actually is!

Okay. What’s about invisibility? Let’s imagine light rays are being bent even more, such as in Fig. 2:

Then the light wouldn’t even reach us! It means that we will never see the object. It will be invisible to us. To bend the light in such a manner we need something more effective and handy than a desert with hot sand.

Small nanotubes, giant strength.

We shall change the sand with carbon nanotubes. They are truly tubes made from carbon with diameter of few nanometers. They have ideal properties for our application. In Fig. 3. they are presented. Carbon nanotubes can be single-walled or multiwalled. Single-walled carbon nanotubes consist of single layer of carbon atoms, whereas multiwalled nanotubes can be considered as several single-walled nanotubes loaded one into another.

What are those ideal properties? We’d like a material which will very rapidly heat the air around it. Carbon nanotubes are very good both electrical and thermal conductors (better than metals) and give away the heat to the air very well. Using nanotubes we can electrically heat the air in controlled manner. Additionally from nanotubes we can get very light and strong materials, which moreover are transparent. This is truly a perfect material for Invisibility Cloak.

Let’s rock the light!

Simply heating the air is not effective enough even using carbon nanotubes. The light will be bent a little but this is not what we are expecting. In order to increase the bending angle we need to… play a little bit of music. It’s just like a snake charmer plays the music to make snakes bend and dance.

To make the light bend we need (as we have seen) layers of thick and thin air. Such layers arranged one after another which move ahead is the sound. What we hear is a disturbance of air (the sound). We hear that the air is alternately thicking and thinning itself.

We can make sound out of nanotubes again. To get it we shouldn’t heat it constantly but just for a moment regularly after some time. After each dose of heat the air will temporarily thin out. During the periods when we don’t heat the air will cool down and become thicker again. We get alternately thicker and thinner air – a sound. Using the sound we furthermore bend the light.

Such double effect (heating and sound making) can redirect the light rays by few degrees. It is sufficient to be invisible to observer which is far enough from us.

There already is such a material! In Fig. 4 there is a sheet made of carbon nanotubes (in the center) connected to electrical current generator. In the left picture the sheet is visible, but if we turn the power on the sheet in the right picture is invisible!

To sum up

Today we’ve exploited an optical illusion to get another idea how to make Invisibility Cloak. That’s not all! In the next post we well present yet another way to get yourself invisible. We’d like to know, what do you think in the comments. Next post in 2 weeks time!