}

Catching reflexes

2008/05/01 Álvarez Busca, Lucía - Elhuyar Zientziaren Komunikazioa Iturria: Elhuyar aldizkaria

If Narcissus had a mirror, he would not die on the banks of the river as he saw his image reflected in the waters of the river. If he had a mirror. But Narcissus did not think he could create a portable surface that would return his image. Yes, however, to a lot of clear heads.
Catching reflexes
01/05/2008 | Alvarez Busca, Lucia | Elhuyar Zientzia Komunikazioa

(Photo: © A. Steiner)
In nature there are surfaces that return our image, such as water and some metals. In both cases, in order to visualize a clear image, the surface must be quite smooth. 4,000 years ago it was seen that when polishing the metals good reflective surfaces were obtained. Looking for better reflections, the Chinese discovered that they mixed the metals of nature and polished a copper and tin alloy, obtaining a suitable reflective surface. Thus were born the bronze mirrors. Since then, mirrors of very different materials, shapes and sizes have been developed, adapted to the needs.

As the Chinese, Greeks, Etruscans and Romans developed the mirrors softening the bronze. For centuries bronze was the most important material for creating mirrors. So it was in modern times until two Muran artisans changed it and got the first glass mirrors. The work of these Muran artisans was undoubtedly the greatest revolution in the manufacture of mirrors.

The Murano Revolution

The basic idea was the same as the background: use metal to get reflections, but in much smaller amounts. Muran artisans began to work glass centuries ago. Two of them obtained a soft and flat glass, and on their back applied a thin layer of a metal with reflective capacity. It was a real revolution.

It became a very exclusive product. On this way of making the mirrors there arose a guild in Murano, for whose manufacture glass and metal composition were secret; if the secret was counted it was punished with the death penalty.

In high-precision mirrors, the metal layer is applied to the support, as smooth and fine as possible.
N. Press releases

Since then, this has been the basic technology of mirrors: polished glass and a metal layer. However, the metals used in the manufacture of the reflective coating have changed. XVIII. In the twentieth century mercury was used, as it is a metal with large reflective capacity. But due to the toxicity of mercury, those who made mirrors suffered vibrations, depression, confusion, insomnia and memory loss, among others. Therefore, over time, mercury was replaced by other metals such as gallians or Indians.

XIX. In the eighteenth century a new revolutionary step is taken in the manufacture of mirrors, beginning to use silver. German chemist Justus von Liebig discovered that silver was very appropriate. It returns 97% of the light it receives between red and green light, that is, most of the visible spectrum. For these properties, in 1857, Jean Focault first used silver for telescope mirrors. Currently, most of the mirrors in the houses are made in silver, although a few are made in copper or aluminum.

Faithful reflection

The common mirrors have the same structure as those invented in Murano. In precision mirrors, however, the structure is inverse. To prevent the glass from diverting light, the metal layer is applied to the mirror holder. Since the light does not have to cross the glass, the rays are not diverted by it.

Such mirrors are used on telescopes, for example. In fact, the rays of light projected by the stars barely reach the ground. In order for these stars to be seen as best as possible, a mirror capable of capturing large amounts of light is necessary.

The polishing of the metals allows its reflectance.
(Photo: P. García Sánchez/
www.flickr.com/
photos/Lordferguson)
There are other problems. The temperature should cause a small expansion in the support of these mirrors. This is why vitroceramic materials are used. In addition, to reflect as much light as possible, these materials must be dielectric, i.e. non-conductive. In short, light is an electromagnetic wave, and the conductive supports would absorb some of those waves. The most used supports are materials such as Pirex and Zerodur.

A perfect mirror theoretically reflects 100% of the light it receives. At the moment, the best dielectrics reflect 99,998% of the light for different wavelengths. For example, lasers of certain colors reflect them in a very concrete way.

In addition to the right materials, the mirror should be smooth to be precise. In the polishing operation silicates are used, since the glasses are composed of silicate and are corroded with products of similar composition. First, with highly corrosive products, silicon oxide glass is shaped. To bring a little closer the necessary shape is used aluminum oxide and finally an iron oxide to leave the surface as smooth as possible. This process has been done manually for a long time. And although very precise smoothness is now achieved through machines, in many cases the final polishing is done manually.

Non-fractional mirror

They also invented a parabolic mirror without polishing: a liquid mirror. When turning a liquid, its surface takes the shape of the parable. In them, as in solids, a metal is used to reflect, but as the name suggests, the metal is in a liquid state.

The material called "zerodur" is dielectric, with a low temperature expansion. Thus, no errors occur in the island of light.
N. Press releases

Mercury, the only liquid metal at room temperature, is used for liquid mirrors, but it is also being investigated whether indio-gallium eutectic alloys can be used for this work. The liquid metal is poured into a bowl that rotates at constant speed. The metal surface is expected to stabilize, and the liquid surface adopts the shape and capabilities of a mirror.

High-pressure air injectors are used to turn the mirror. Unfortunately, these structures cannot withstand very heavy structures, which limits the diameter of the liquid mirrors that can be obtained. Therefore, it is necessary to build a container with a limited weight and use as few metals as possible. Parabolic containers are constructed, allowing the use of very thin layers of mercury of only 1 or 2 mm.

They have a problem. Liquid mirrors are necessarily cenital, that is, they look up and cannot move through that position. They do not support oscillations nor can they be moved from place to angle. This limits the use of these mirrors. And, at the same time, the size of the mirror is a limit. Theoretically, liquid mirrors up to 15 meters can be obtained, although these measures have not yet been reached. The largest liquid mirror currently available is in Canada. The Large is a 6-metre diameter Zenit Telescope primary mirror.

Dividing, higher

The larger the mirrors, the more accurate the images are. But, as in liquids, in solids there are technical limitations for the manufacture of large mirrors.

In liquid mirrors the mercury rotates at constant speed until the surface stabilizes and takes the form of a mirror.
P. Hickson
The greatest limitation is produced by the reflective material. For the manufacture of high precision mirrors it is necessary to glue the aluminum particles on the surface to get a layer as thin as possible. To do this, work in a vacuum environment. The largest vacuum bells currently existing have a diameter of 8.5 meters. This is the theoretical limit of the diameter of the mirrors. The mirrors closest to this limit are the Great Binocular Telescopes of Arizona. This telescope uses two 8.4-metre diameter mirrors, practical limit. At least in one-piece mirrors.

But you can also make segmented mirrors. In fact, the union of many 'small' hexagonal mirrors exceeds eight metres in diameter. Thus, each segment is within the limit of the aluminizing machine. Currently, the largest segmented mirror is being built in the Canary Islands. The primary mirror of the Gran Telescopio de Canarias will have a diameter of 11.3 meters and will consist of 36 hexagonal mirrors.

The largest mirrors in the world.
L. Alvarez

Segmented mirrors will allow to build larger and larger mirrors. At the European Southern Observatory (ESO), together with European companies and universities, they want to build a segmented 100-metre diameter mirror to be used on a telescope. The project, known as the ELT (Extremely Large Telescope), would undoubtedly be the largest mirror of a telescope. And, of course, the largest in the world.

Solar mirrors
Mirrors are used for many things. Also for energy. There are ways to obtain electricity through reflection of the Sun's rays. It receives the solar rays by spherical mirrors and concentrates them in a specific point, the focus of the mirror. At this point, of course, solar rays produce a lot of heat. Heat evaporates water; steam moves turbines that, in turn, generate electricity.
Energy companies, through many mirrors, form a large spherical mirror. These mirrors are heliostats, that is, they follow the movement of the Sun and constantly project the rays of the sun. For example, in Seville there is already a company that gets electricity in this way, and currently gets the same energy as 6,000 households.
Behavior of anti-mirror rays
When the rays of light collide against the mirrors, they present different behaviors depending on the shape of the mirror. In flat mirrors, when light rays collide with the mirror, they are reflected. But not in any way. These rays reflect the mirrors with the same angle they carry when they collide with the mirror. But the same does not happen in spherical mirrors.
(Photo: G. Roa)
This is perfectly explained by the example of parallel rays. The rays of light parallel to each other deviate as they hit a flat mirror. But although they deviate, being the flat mirror, they all deviate at the same angle. Thus, the mirror projects the image without deformations.
The same does not happen with spherical mirrors, that is, with concave or convex mirrors. When the rays of light collide against the mirror, the spherical mirrors reflect at another angle, the rays lose their parallelism and the image they project is deformed. This is the image you see when you reflect on a spoon or the comic image returned by the exhibition mirrors.
Behavior of parallel rays in different types of mirrors.
(Photo: L. Alvarez)
Concave and convex mirrors also facilitate many tasks. Unlike flat mirrors, convex mirrors obtain images that transcend their boundaries. An example of this is the mirrors that are placed on road crossings to see if there is another car.
Concave mirrors do the opposite and concentrate instead of enlarging the image. When striking the light rays against the mirror, if the mirror shape is right, all the rays are concentrated at the same point. This point is called mirror focus. In the focus the image reflected by the mirror would be projected.
Alvarez Busca, Lucia
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