SILICON – The Miracle of Advanced Technology _ PART III (WHY is the glass Transparent?)

If I would be asked to define what makes material science the most important knowledge necessary for humans to continuously develop the technology and everything around us, I would only say two words: TEMPERATURE VARIATION.

That’s in fact valid for the entire universe, not only for the Planet Earth. The Temperature really defines everything. If the whole universe would have a constant temperature so that the absolute thermodydamic equllibrium would be reached and the entropy no longer applies, then nothing would exist anymore. The reason why there is life on Earth is exactly this, becasue the Temperature varies and additionally on our planet its variations are exactly at the right ranges so that it allows life and things to exist. The same happens with materials, they exist in so many types and chategories that it is practically impossible to quantify their exact number. The Temperature creates miracles. All what is going on in material science is the result of different processes from which different materials are obtained as the final outcome generated by playing with temperature. By constanly doing this, evolution has made us ending up with the today’s advanced technology and there is no end point to this. Playing with it, by heating and cooling different stuff, we can get extraordinary results which can completely change our lifestyle. It happens all the time and it is the case for the extraordinary element called Silicon, as well. Looking at sand in its natural form we don’t put much value on it, on the contrary we even try to get rid of it as much as possible, mainly when we go to the beach and the wind is blowing it all over.

Sand is an opaque stuff made of tiny bits of stones having different collors and shapes. Yet by melting and cooling in a process with specific steps involving optimal temperature ranges something amazing happnes. It transform itself in a new material called Glass which in turn has a absolutelly remarcable property that of being transparent.

So then why is it that glass has this apparently miraculous property of transparency? How is it that light can travel through this solid material at all, while most other materials will not allow it? After all, glass contains all of the same atoms that make up a handful of sand.

Why in the form of sand should they be opaque and in the form of glass transparent and able to bend light?

Glass is made of silicon and oxygen atoms, as well as a few other sorts. Within every atom there is a central nucleus, which contain protons and neutrons, surrounded by varying numbers of electrons. The size of the nucleus and the individual electrons is tiny compared to the overall size of the atom. If an atom were the size of an athletics stadium, the nucleus would be the size of a pea at its centre, and the electrons would be the size of grains of sand in the surrounding stands. So within all atoms – and indeed all matter – there is a majority of empty space. This suggests there should be plenty of room for light to travel through an atom without bumping into either an electron or the nucleus. Which indeed there is. So the real question is not  ‘Why is glass transparent?’, but ‘Why aren’t all materials transparent?’

Inside an atomic stadium, to continue the analogy, the electrons are only allowed to inhabit certain parts of the stands. It is as if most of the seats have been removed and there are only certain rows of seats left, with each electron restricted to its allotted row. If an electron wants to upgrade to a better row, it has to pay more – the currency being energy. When light passes through an atom it provides a burst of energy, and if the amount of energy provided is enough an electron will use that energy to move into a better seat. In doing so, it absorbs the light, preventing it from passing through the material. But there is a catch. The energy of the light has to match exactly  that required for the electron to move from its seat to a seat in the available row. If it’s too small, or to put it another way, if there are no seats available in the row above (i. e. the energy required to get to them is too large), then the electron cannot upgrade and the light will not be absorbed. This idea of electrons not being able to move between rows (or energy states as they are called) unless the energy exactly matches is the theory that governs the atomic world, called quantum mechanics.

The gaps between rows correspond to specific quantities of energy or quanta. The way these quanta are arranged in glass is such that moving to a free row requires much more energy than is available in visible light. Consequently, visible light does not have enough energy to allow the electrons to upgrade their seats and has no choice but to pass straight through the atoms. This is why glass is transparent. Higher-energy light, on the other hand, such as UV light, can upgrade the electrons in glass to the better seats, and so glass is opaque to UV light. This is why you can’t get a suntan through glass, since the UV light never reaches you. Opaque materials like wood and stone effectively have lots of cheap seats available and so visible light and UV are easily absorbed by them. Even if light is not absorbed by glass, moving through the interior of an atom still affects it, slowing it down until it emerges from the other side of the glass, when it speeds up again. If the light strikes the glass at an angle, different parts of the light will enter it and emerge from it at different instants, forcing them to travel momentarily at slightly different speeds.

This momentary difference is what causes the light to bend, or ‘refract’ and it is this that makes an optical lens possible, with the curvature of the glass resulting in different angles of refraction at different points along its surface. Controlling the curvature of the glass means we can magnify images, which allows us to make telescopes and microscopes, and, for those of us who wear glasses, to see. It also, and perhaps more fundamentally, turns light into a subject for experimentation itself. Through the centuries all glass makers must have noticed that glass could create mini-rainbows on the wall as sunlight hit it at particular angles, but no one could explain the cause, except to state the blindingly obvious, which is that the colours were somehow being generated within the glass. It wasn’t until 1666 that the scientist Isaac Newton realized that what was blindingly obvious was blindingly wrong and came up with the real explanation.

Newton’s moment of genius was to notice that a glass prism not only turned ‘white’ light into a mixture of colours, but could also reverse the process. From this, he deduced that all of the colours created by a piece of glasswere already in the light in the first place. They had travelled all the way from the sun as a ray of mixed light only to be split up into their constituent colours when they hit the glass.The same thing would happen if they hit a drop of water, too, since this was also transparent. At a stroke, he had for the first time in history managed to explain the main features of the rainbow. The satisfying explanation of an atmospheric phenomenon using a laboratory experiment showed the power of scientific reasoning. It also showcased the role of glass as laboratory accomplice in unravelling the mysteries of the world. This role was not limited to optics. Chemistry was transformed  by glass perhaps more than any other discipline. You only have to go to any chemistry lab to see that the transparency and inertness of the material make it perfect for mixing chemicals and monitoring what they do. Before the glass test tube was born, chemical reactions were performed in opaque beakers, so it was hard to see what was happening. With glass, and especially with a new glass called PYREX that was immune to thermal shock, chemistry as a systematic discipline really got going.

In 1985 was the official launch of the first range of glass freezer storage containers. Whilst all households became equipped with microwave ovens and freezers, Pyrex® brand launches a line of storage containers that can be used both in the freezer and oven. The company designs dishes that have a cubic form in order to fit in the microwave and gain space in the cupboards.

PYREX is a glass with boron oxide (B2O3) added to the mix. This is another molecule that, like silicon dioxide (Si2O), finds it hard to form crystals. More importantly, as an additive it counteracts the tendency glass to expand when heated or contract when cooled. When different parts of a piece of glass are at different temperatures, expanding and contracting at different rates, stresses build up within the material as the different parts of the glass strain against one another. These stresses cause cracks to grow and ultimately shatter the glass. If this happens in a vessel containing boiling sulphuric acid, such a failure can maim and kill. The discovery of borosilicate glass (PYREX is a trade name) put a stop to thermal expansion, and so to the stresses associated with it. It released chemists to heat and cool their experiments as they wished, to concentrate on chemistry and not the potential dangers of thermal shock.

Glass also allowed them to bend glass tubes within the laboratory with the aid only of a blow torch and to construct complex chemical equipment such as distillation vessels and gas-tight containers much more easily. Gases could be collected, liquids controlled, chemical reactions allowed to do whatever they liked. Glass equipment is the workhorse of the chemist’s world – so much so that every professional chemical lab has a glassblower in residence. How many Nobel Prizes did this material make possible ? How many modern inventions started life in a test tube?

Whether the relationship between glass technology and the 17th-century scientific revolution really is a simple case of cause and effect is an open question. It seems more likely that glass was a necessary condition rather than the reason for it. However, there is no doubt that glass was largely ignored in the East for a thousand years. And during this time, glass revolutionized one of Europe’s most treasured customs. While glass had been used by the rich to drink wine for hundreds of years, most beers until the 19th century were drunk from opaque vessels such as ceramic, pewter or wooden mugs. Since most people couldn’t see the colour of the liquid they were drinking, it presumably didn’t matter much what these beers looked like, only what they tasted like. Mostly, they were dark brown and murky brews. Then in 1840 in Bohemia, a region in what is now the Czech Republic, a method to mass-produce glass was developed, and it became cheap enough to serve beer to everyone in glasses. As a result people could see for the first time what their beer looked like, and they often did not like what they saw: the so-called top-fermented brews were variable not just in their taste, but in their colour and clarity too. Not ten years later, though, a new beer was developed in Pilsen using bottom-fermenting yeast. It was lighter in colour, it was clear and golden, it had bubbles like champagne – it was lager.

This was a beer to be drunk with the eyes as much as with the mouth, and these light golden lagers have continued in this tradition ever since, being designed to be served in a glass. How ironic, then, that so much lager is drunk from an opaque metal can, meaning that the only beer uniquely identifiable for it visual appearance is the epitome of opaqueness, a beer in the old pre-glass tradition, Guinness. The move to serving beer in glasseshad another unexpected side-effect, According to the UK Government, more than 5000 people are attacked with glassesand botdes every year, costing the health service more than L 2bn (pounds) to surgically repair the  injured. Although many alternative plastie materials for serving beer in bars and pubs have been tried, materials which are both transparent and tough, they have never gained acceptance.

Drinking beer from a plastic cup is a completely different experience to drinking from a glass. Not only does plastic taste different, but it also has a lower thermal conductivity, a property that makes it feel warmer than glass, reducing the satisfaction of drinking an ice-cold beer. Plastic is also much softer than glass, so plastic beer cups soon become tarnished, scratched and opaque. This masks the clariry of the beer but it also affects our perception of the cleanliness of the vessel. One of the great attractions of glass is that its shiny bright appearance makes it seem clean even if it isn’t, a collective deception we all accept in order to avoid thinking too much about using the same glass that was in a stranger’s mouth perhaps only an hour before.

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