Naughty light

2007/01/01 Roa Zubia, Guillermo - Elhuyar Zientzia Iturria: Elhuyar aldizkaria

• Fisika

The example of the glass and pencil has been used many times to talk about light. Putting the pencil into the glass, without completely immersing itself in the water, clearly shows the effect of the refraction: it seems that the pencil is broken from the point of entry to the water. It is an optical effect, the pencil is not broken, but the light rays change direction at that point and the eye sees the light (not the pencil). It is a basic effect.

Basic but of great importance. Light acts like this on any transparent object and anyone working with the light should take into account the effect. The light refracts. But it does not refract the same in all materials; if oil is placed in the place of water, it seems that the pencil is still broken, but the angle of inclination is not the same in both cases. There are materials that cause to light a greater angle than others, a different refraction. Physicists use the refractive index to express it: the higher the number, the more the direction of light tilts the material.

And, of course, the lower the refractive index, the less the direction of light worsens. But where is the limit? The index of material not able to tilt the light will be zero. But the zero value does not have to be a limit, at least in theoretical physics. Zero is not a limit, but the beginning of the negative number field. This idea can be applied to the refractive index, is there no material with negative refractive index? It would be a material that tilts the light to the other side.

Negative index

Upon entering a material with a negative refractive index, the rays instead of advancing in the direction of light, backed back. These materials were sought and found. Or rather, they did.

They had to understand the relationship between light and matter. There are materials that allow the passage of light --glass, water or air- and other non-wood, milk or water vapor. Some materials are transparent and others opaque. (And others are intermediate because they let part of the light pass through.) The question is why that happens.

Light is an electromagnetic wave. This means that it has two components: an electric field and a magnetic field. Letting these two areas pass is letting the light pass. There is the key.

They are two characteristics: the electric field on one side and the magnetic field on the other. The question is how easily each of them goes through the material. Physicists use two parameters that measure it: electrical permittivity and magnetic permeability, respectively.

Electrical permittivity indicates the response of a material when it meets an electric field. If positive, all electrons in the material move to the side that pushes the zones and if negative to the opposite side. In most materials it is usually positive, but there are materials with negative permittivity, even in nature.

The same goes for magnetic permeability. This parameter represents the response of a material to a magnetic field. If it is positive, the magnetic response is aligned with the field, that is, it is placed in the direction and direction according to the field, and if it is negative in the direction of the field, but in the opposite direction.

A little math

Taking these two parameters into account, you understand what happens to light when you encounter a material. The refractive index can be easily calculated from these two parameters using the following simple formula: n = www.euskaltel.com

The symbols n,{ y ã, express the refractive index, electrical permittivity and magnetic permeability, respectively.

The formula is simple, it is the square root of a product, where the key to the variations of the index is found. When the square root gives a real number, the refractive index can be positive or negative; and when the square root gives an imaginary number, the refractive index concept makes no sense, that is, that material is opaque.

In short, from the point of view of the symbols of both parameters, there are four options, since these are two parameters and each parameter can be positive or negative. However, as for light response, materials can be classified into three groups.

The materials of the first group present both positive parameters. (Therefore, the inside of the square root is also positive, and the root with physical meaning, refractive index, is positive.) As a result, they let the light pass, that is, they are transparent. The simplest examples are glass, water and air (examples above). Of course, polymethacrylate is very common in current technology for the manufacture of transparent objects. And it should be noted that the vacuum is also found in this group, that is, the empty space, which from the point of view of the transmission of light can be considered material, has positive permissions and permeabilities --they are not zeros -, and therefore is transparent. So we see the sun and the stars, among other things.

The elements included in the second group have a single positive parameter, the other is negative. It can be permissible or permeable, it does not matter. Being a negative, mathematically, refractive index is the square root of a negative number. This has no physical meaning and, in fact, that material is opaque, does not let the light pass. Of course, nature is full of these materials. Silver, gold and many other metals have negative permittivity and positive permeability. They are opaque.

In the materials of the last group, both parameters are negative, permissible and permeable. In short, they present a "reverse" behavior with electric and magnetic fields. Both. For these materials, the root with physical meaning, the refractive index, is negative. In these materials the light, so to speak, turns back. The problem is that such materials are not found in nature. But they have been done artificially, they are metamaterials.

Metamaterials

Today they exist, but until recently they were only an ancient theoretical concept. Absence of materials in nature with electrical permittivity and negative magnetic permeability.

In 1968, the Russian physicist Victor Veselago announced the behavior of these materials if they exist. He predicted the concept of negative refractive index. However, I did not know how these materials could be made. The British John Pendry made a proposal in 2000.

Pendry's idea was to join two unclosed copper rings, one inside the other and united by a conductive thread. It is a two-dimensional structure, but if many of your copies are organized into a three-dimensional network, the result can be a metamaterial. Allows and negative permeability. The idea was from John Pendry, the material they produced at the University of California, in San Diego, and they were able to confirm that Veselago's predictions take place in reality.

In practice, what?

Metamaterials technology is in its first steps. At the moment, it has only been tested in physics laboratories. In addition, with the visible light they have not achieved this effect, since the permissible or negative permeability for waves greater than the microwave is lost. To achieve the effect of negative refraction other metamaterials must be performed.

However, the results are not bad. One example: they claim that perfect lenses can be made. Perfect lenses would do what conventional lenses do, but without causing the aberration of light. They would be flat and thin lenses, as they would not need curves to drive the beam of light. Therefore, very light weights could be made, which is a revolution in the world of glasses, telescopes, etc. Of course, to do this you have to look for metamaterials that work with visible light.