# How can a body be transparent?

How can a body be transparent? I need the theoretical explanation for the same. Has anybody succeeded in doing that practically?

• This might be a better fit for Physics.SE; if you had a specific fictional universe in mind, that would be one thing, but you appear to be asking about reality, although you tagged it as 'Magic' and 'Magical-theory.' If the tags are correct, the answer is 'Magic', but it looks like you are asking for a real-world explanation.
– Keith H Weston
Jun 16, 2012 at 16:16
• Magic... Aka Quantum Mechanics. Water and glass are more transparent than metals because they don't have as many electrons "stealing" the light and deflecting it away at other electrons causing the photons to bounce all around before getting through. A perfectly transparent material would need to either allow light to travel cleanly through without "dirtying it" or if you'd like something to put on (like Harry Potter's coat) you would want something to glide the light along and out at the other end as if id did go right through you. (see below comments for details) Jun 20, 2012 at 5:23

It is possible - so far theoretically, and yes, there are groups which do their research in this topic.

The key factor is called Metamaterials. Briefly, metamaterials are materials which have effective permeability or permittivity lower then 1. In electromagnetic theory it means that refraction index is negative and group velocity is higher than speed of light (be aware of that we are not talking about velocity of information, we are not breaking the causality law).

How it can be used? You have an object which you would like to make invisible. You will put a layer of this metamaterials around it. If an electromagnetic wave (light, RF, whatever) enters, it will be shaped around the object and will leave the metamaterial layer on the other side. Since the group velocity is higher than the speed of light there will be no delay.

In practice, there are several issues which are not solved yet. We can do it just for very narrow band of frequency (ok, you are invisible for radar but not for human sight). So far we can make it just for low frequency (around 20 MHz). We need power supply for the metamaterials and some other issues.