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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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They were first to do this in the 700 nm range but the article state that previously this could only be done in the 1400 nm range. I guess 700 nm is significant because it is the start of the visual spectrum. 700 is red i think.
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people can wear defense cloaks to prevent the effect of the military's microwave guns (http://www.telegraph.co.uk/news/main.jhtml?xml=/n ews/2004/09/19/wirq319.xml)?
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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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Write it in Java.
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
Short version: light travels at different speeds through different substances. It's faster in air than it is in water or glass. When it strikes a boundary between two substances -- say, air and glass -- at an angle, it will turn slightly, because one edge of the beam hits the new substance sooner than the other, and will slow down (or speed up) sooner.
This is why you sometimes see two of the same fish when you look at the corner of a fish tank. The light gets bent as it travels from water to glass, and again from glass to air, resulting in two paths from the fish to your eye. This is also how lenses work.
So that's refraction. The refractive index is essentially a measurement of how much it bends when pssing into that substance.
(Honestly, I learned about refraction in third or fourth grade. What do they teach in schools these days?)
Refractive index basically describes how fast light moves through a material. That's the "speed at which they propagate" part of the quote you cited - in materials with a high refractive index, which you might think of more "optically dense" or preventing more barriers to the "movement" of the light, light travels more slowly than it does in materials with lower refractive indices.
:-)
When you have two materials with different refractive indices up against each other, light bends by some angle (the angle depends on how close the refractive indices of the two materials are). I'm sure you've seen the effect where you put a straw or a pencil into a partly-full glass of water (if you haven't, go try it) and the straw/pencil appears to be bent - this property of refractive indices is what's causing this phenomenon.
Basically, a negative refractive index changes/reverses the angle at which light bends, which can lead to some pretty funky optical effects. If you go to the wikipedia page on "Metamaterial" there's a diagram indicating this concept.
Does that help? It's not a precise technical definition by any means, but then, I don't think a precise technical definition is what you were asking for, hm?
Never mind what this does to the coin-in-the-bowl-of-water trick!
Sheesh.
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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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These metamaterials have a long and interesting history (many posts here on slashdot and elsewhere) -- long because they were predicted a while ago by Veselago, and interesting because of the recent interest due to Pendry's production of workable devices in electromagnetic fields. There are even meta-materials being produced for acoustics problems, too.
However, what I'm really looking forward to is a Somebody Else's Problem device -- this will make all of the other foophraw unnecessary.
> 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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C is the speed of light in a vacuum. Light in air travels at C velocities. Light in liquids travel even more C. While there are legitimate concerns about the possibility of anything, light included, traveling faster than C your question is irrelevant as the light is not 'traveling faster than itself' but rather traveling faster than it would in a vacuum. It's like saying 'The plane's Cruise speed is 300 MpH, how can it cruise faster than itself?'
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When you get to high school you'll cover this stuff in physics class.
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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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As I understand it, post secondary education has been completely free for the past 200 years.
I could be wrong, but if I'm understanding the physics properly, then there's a substantial barrier to using this technology for invisibility: all these meta-materials are highly dispersive, so the effect is unlikely to work over any significant range of wavelengths.
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Actually, it doesn't matter how many cones you have; it matters what range of frequencies they cover (for purposes of invisibility). The cones I mentioned are optimal at the points specified but cover the entire "visible" range. The only advantage a tetrachromat would have for this cloak is if their fourth cone extended the range of their visible frequencies (which it does tend to do). However, you could also have only 2 cones and still have a visible range outside of what is considered normal, so being a tetrachromat is neither necessary nor sufficient.
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I thought Jethro Tull invented the aqualung.
http://ol.osa.org/abstract.cfm?id=119886 You have to keep in mind that before Arxiv.org papers (or any other pre-print archives) appear in a journal, you can't guaranteed that they have passed the peer-review process.
Yeah, the metaphor that helped me understand was pointing at an object many light years away with a very bright laser. If you wave the laser back and forth, the dot on the distant object would appear to move faster than light, but the dot isn't an object, it's the point at which the laser beam hits the object. The laser itself is moving at c and no more.
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It'll never happen. Respectable intellectuals hate to hear things that aren't politically correct, and as such tend not to test them.
110100 1101000 1101000 1100110 0 1101111 1101000 1100011 1
"Dolling 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.
This value drops to zero at 760 nm and 800 nm, and becomes positive at longer and shorter wavelengths. Previously, the shortest wavelength at which a negative refractive index had been demonstrated was 1400 nm. "
how is this possible? fractional indices would imply that the light is going faster than light in a vacuum. i would expect negative index materials to have indices of less than -1 and no material to be able to have anything between -1 and 1.
Metamaterials and the concept of negative index of refraction are likely the cold fusion equivalents of this decade...
There are several weak points in this whole business of "Harry Potter cloaks" where physicists with little experience in electromagnetics (and even less in radar cross section reduction) go astray. To list but a few points:
Irrelevance of group velocity
It has long been known that effects like anomalous dispersion in resonant media can render classical group velocity concepts irrelevant. Several authors seem to lack an understanding of the inherent assumptions when equaling the group velocity with a power or information transfer speed. Thus an interpretation leading to an "equivalent" negative index of refraction can be misleading.
Bandwidth
The bandwidth of these materials is inherently small. There is also often a significant loss as well.
Misuse of models
The assumption of monochromatic and plane waves interacting with an infinite structure will be like pressing a square peg into a round hole when dealing with some cases. For example, it is well-known that a simplistic plane-wave model is invalid when dealing with lossy materials (apart from normal incidence).
Publications in out-of-field journals
It is clear that a lot of the metamaterial material has been published in journals that are outside the typical antenna or microwave area, such as Nature, Science, and Phys. Rev. This could potentially lead to deficient papers slipping through, due to lack of a proper review. An example of something that hopefully would have been curbed in an IEEE journal is a Phys. Rev. paper [*] that showed a transmission vs. frequency plot with a dynamic range of 1600 dB! (The range of scale of the size of the universe compared to the Planck length is dwarfed by this...). There are numerous examples of publications without even the most basic sanity checks performed by the authors and the reviewers. The situation has been bad enough for the microwave field, now it is unfortunately spreading to optical frequencies.
[*] R.W. Ziolkowski and C.-Y. Cheng, "Existence and design of trans-vacuum-speed metamaterials", Phys. Rev. E, 68, 026612, 2003.
Peer review endangered
The field of metamaterials has now grown to such a volume that a wholly separate sub-science or "sect" with its own special issues and conferences, etc. has formed. There is an inherent problem with this, since the peer review process will be endangered. The people most knowledgeable within the subject are by definition those that are active within the subject, and fewer outside reviewers will be used after a while.
"Publication by news releases"
Several of the groups within this field are heavy on marketing their results as revolutionary. In the present "publish or perish" environment it is very important to secure funding, and gullible grant-givers are abundant...
It'll never happen. Respectable intellectuals hate to hear things that aren't politically correct, and as such tend not to test them.
Three points...
1. Not only won't it happen, it CAN'T happen. IQ tests are culturally biased. Comparing different cultures by measuring IQs has to many uncontrolled variables to provide meaningful results.
2. IQ test don't measure anyting other than ability to take IQ tests.
3. Don't make the mistake of thinking that just because something is politically incorrect it isn't also morally or ethically incorrect, or just plain vile and wrong.
People who've had artificial lenses replace their own (because of cataracts or some other ailment) apparently can see the near ultraviolet which is said to be blueish-white. The reason for this is because the lens blocks these wavelengths but the cones are sensitive to them. Perhaps the wavelengths are blocked because they could be harmful to the retina or perhaps its just one of those biological quirks.
Also if you make the source REALLY bright then apparently human vision can extend a very short distance into the near infrared as a very bright near infrared source will excite the rods or cones (not sure which it is) a tiny amount.
In case anyone is wondering what a negative index of refraction would look like, this is a very good start:
5
http://www.opticsexpress.org/abstract.cfm?id=8832
Examples (including avi's) rendered in Povray, the free raytracer. One of the authors is Chris Hormann, one of Povray's main code contributors.
I don't feel like it...