Sci-fi of Today: Invisibility Cloak Part 7
Previously we discussed about our next method for invisibility which required the knowledge of a science field known as transformation optics and the use of synthetically made materials known as metamaterials. We quickly went over the metamaterials last week and explained that they could be used to bend light and therefore make someone invisible. This all sounds good and fun but unless you have a little background in optical physic, you might as well consider it all magic. Now magic is cool and everything, but this is science and therefore I’ll try to make a quick review of basic optics so anyone at least has a general idea of what is going on, keep in mind this is not a physic crash course, I really couldn’t care less if you can calculate the angles of diffraction of light through different medium. We will begin with explaining a bit what diffraction is and then we’ll see how it applies to our concept of invisibility.
Let us begin by understanding what diffraction is. In a simple form, diffraction is the way light bounces, curves and overall changes direction when it encounters an object. Light can be viewed as waves of an ocean, it acts in very similar ways when it moves around. If you have light going into a small opening, the light won’t simply come straight out of the opening like an arrow, instead it will disperse at the other hand. This example can more easily be visualized if we use different scenarios. Imagine a car that comes out of a tunnel and water coming out of a river. A car that comes out of a tunnel will continue it’s trajectory straight forward and there won’t be any pieces of car anywhere around the exit, just one car continuing in a straight line. On the other hand, water coming out at the end of a river will spread itself. Most of the water would continue in a straight path, but there is a portion that would flow to the sides and expand towards every available direction. Light acts the same way as water does, it acts as a wave.
Here is a visual interpretation of what this means:
As you can see, the light has to go through a tight hole to go to the other side, after entering the hole it exits in a wave pattern and propagate everywhere. Now that you know a little bit more about how light behave, we can go into the thick of the subject, diffraction.
An easy way to explain diffraction would be to continue from our previous example of waves. Diffraction is anything related to the way light direction is affected when it encounters an obstacle, so when in the animated example above light encounter a wall with a small opening, the small opening makes it an obstacle for light to go through and therefore diffraction occur. In our example the light was projected on the other side in a wave pattern, but it must be understood that by doing so, some part of the light was “bent” in the process. Portion of the light curved around the edge and changed trajectory from their original point. Another way to have light change trajectory is to have it go through an object.
Now this might sounds weird at first “how can light go through an object”, but then you will realize that there are many object you know which light easily go through. Think of Glass, Water, certain plastic; those are all daily life item that you can easily see light going through. The best obstacle we can use to help us understand how things work will be water, because we are all familiar with it and you can test at home with it pretty easily. I’m sure no one will try to test that you can see an object through water, it is pretty common knowledge, but it is important to notice too that object you see through water are distorted. Have you ever noticed that when you look down at a straw in a glass of water (or most other drink since they are mainly made of water) the part of the straw in the water looks bent? In reality the straw obviously doesn’t change shape, it is instead only the light coming from the water that is bent. Because the air and the water have different consistencies the light will bend at the edge of the medium. If you were to put the straw into a bath instead and plunge your head under the water, the straw would appear normal once again, it is only once you watch it from the air, above the water, that you would see the weird angle. This phenomenon is caused by the Refractive index of the elements. Now, what does that phenomenon means exactly?
Well light usually travels at the speed of light…or so you would think. In reality light only travels at the speed of light when it is in a vacuum, in complete emptiness. When light travels in air, it actually travels 1.000293 slower than the speed of light. Granted, this changes absolutely nothing from our perspective, but it gets important when we talked about other, denser medium. For example light travels 1.33 times slower when it enters water, this is a change that is starting to be significant enough for us to notice, and as such we do, the difference in speed of the light when it travels through air and water is the reason why your straw appears at an angle. Now obviously this leaves a lot of questions open, and we will answer those, but only in Part 8. The important point of this section is to remember that light is a wave and it can be bent when it encounters obstacles.
Next week we will discuss more about the refractive index of light and the properties of light, we will see how light can be captured and therefore how metamaterial actually works scientifically speaking. Stay tuned!
ZeroGhj signing off