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Material With Negative Refractive Index Created
holy_calamity writes "The race to build a material with a negative index of refraction for visible light has been won by researchers in Germany. The advance could lead to super-lenses able to see details finer then the wavelength of visible light, or the previously predicted invisibility cloak for visible light." From the article: "[The researcher] determined the refractive index of the material by measuring the 'phase velocity' of light as it passed through. His measurements show the structure has a negative refractive index of -0.6 for light with a wavelength of 780 nm [the far red end of the visible light spectrum]. This value drops to zero at 760 nm and 800 nm, and becomes positive at longer and shorter wavelengths."
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Red is ~700 nm and violet is ~400 nm. A typical human can see light from the range of 390-750 nm with the aid of three cones. The three cones are the "red" cone (optimal at 564 nm), the "green" cone (optimal at 534 nm), and the "blue" cone (optimal at 420 nm).
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Wikipedia does a good job describing refraction and the refractive index. You should try to understand refraction before trying to understand the refractive index.
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An invisibility cloak..
For the first time, I may have a real shot at seeing real life naked boobies
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I can understand how this material can make an item stealthy from radars and all. This material can be used to bend / deflect the rays so that they never return to the radar. But the same concept does not an invisibility cloak make. If a cloak deflects light, then the human eye will see a missing spot (Because, unlike the radar, an eye would see everything else around the cloak).
So, for a cloak to be invisible, we need it to pass light from the other end of the cloak. For this, the cloak would need to know the geometrical shape that it has currently, absorb light coming from one end, and forward it to a light emitting object on the other end of the cloak. The problem then will be that the cloak would need to know where the "eye" is to be able to map back and front ends correctly.
Am I talking non-sense here?
If I check another site, I lose my excuse for bashing the poor quality of the article. That's just not an option.
When one talks about a wave propagating through a medium, there are two velocities that one usually considers, the group velocity and the phase velocity. The group velocity is the speed at which energy and information are moving. (This isn't always true, but for most materials it is or is a good approximation.) The phase velocity is how fast a "phase" (a feature like a crest) appears to be moving.
A good way to visualize the difference is to think of a ocean waves hitting a wall at an angle. The speed which with the wave itself is moving is the group velocity, but if you look at the wall, you will see the crests moving along at a different speed. (If you have trouble seeing that, make a little sketch.) There is also a nice Java applet (GPLed!) here, which does a good job of illustrating the difference
I actually achieved this a couple of years ago. But the phone rang and I set it down somewhere, and now I can't find it.
No, he said _faster_
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Furthermore, it doesn't explain what the basic properties of a positive refraction index are (aside from saying that it's normal), let alone what negative indexes could do.
In ordinary optics, refractive index is the ratio of the velocity of light in vacuum (c) to the velocity in the material (v):
n = c/v
Since v <= c, n >= 1 is always true.
But light, being wavelike, has two velocities associated with it: the phase velocity, which is the velocity of an individual crest in a monochromatic light wave, and the group velocity, which is the velocity of a wave packet consisting of many frequencies. Depending on which velocity you care about, and how you deal with wave packets, it appears that you can extend the definition of refractive index in such a way that negative refractive index is meaningful. The discussions of this that I have seen online are uniformly confusing, so I'm not clear on exactly what is going on, although it is clear that negative extended refractive indices do make sense.
One analogy to think about is the conventional definition of resistance: R = V/I. Clearly by this definition resistance is always positive. But if instead you think of resistance as being the slope of the V/I curve, it is clearly possible for a device whose (conventional) resistance decreases with increasing current it is possible to have a slope that is negative, and this can be treated as "negative resistance". Tunnel diodes exhibit this effect.
If one were to be gloriously pedantic about this, one would only use the terms "negative extended refractive index" and "negative extended resistance", because "negative refractive index" and "negative resistance" are confusing oxymorons to the vast majority of people in the world who are at best familiar with the conventional definitions. And in fact, we usually do make this kind of distinction. We use terms like "electric car" because "car" means "internal combustion engine hydrocarbon-powered road vehicle" to the vast majority of people. Therefore headlines like, "New Car Does Not Need Gasline" would be obviously misleading and confusing if they actually meant "New Electric Car Does Not Need Gasoline."
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> Come on doofus, C is the the speed of light. How can light go faster than itself ???
Perhaps it could go as fast as his post went over your head.
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You can't find the phone? It's probably under your invisibility cloak. Try the pager from the base.
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Another way to describe the refractive index is in terms of the square root of the relative permeabilities and permittivities. In a negative refractive index, epsilon and mu are both negative. However, the refractive index is the square root of the product of these two. So they probably just retain the sign on the refractive index to show this important characteristic.
Basically all it means is that light is going to bend opposite of what we would normally expect. Instead of bending towards the interface, light will bend away from the interface. There's no fancy u-turns or anything like that. The negative sign is purely a consequence of the convention by which we choose our cross products when it comes to the vector form of Maxwell's Equations. Normally we use a right-hand convention, but a metamaterial behaves using the left-hand convention. This negative sign is one way of achieving the same effects using the right-hand vector convention.
...here, gives (under metamaterials) a good example of what negative refraction is here
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I would see an immediate use (presuming reasonable cost) in using something like this in camera lenses to combat chromatic aberration. Regular lenses bend light differently at different wavelengths so that the various colors don't focus exactly. With something that has a negative refractive index, the light could be passed through a set of these lenses to get the focal point to a single point.
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