How do optical fibres work?

Everybody's heard of them. They are used to connect your computer to the internet, your sound boxes to your surround system at home, in the cable TV system. To understand why these optical fibres are used, we first have to know how optical fibres work. Unlike other cable-wiring, optical fibres use light instead of electrical signals to transport information. How is it possible that light can transport information? How is it even possible that light can travel through an optical fibre? The answer to these questions is total internal reflection. When, for example, a beam of light (traveling in a glass plate) strikes the glass-air interface, a part of the light is reflected (reflected beam) and a part of the light is transfered through (refracted beam) and escapes the glass plate. This phenomenon is depicted below for a water-air interface.



As the reflected beam stays inside the glass plate, we can see that the problem lies in the refracted beam: all light that escapes the optical fibre causes the light signal to be less strong. When the incident light beam, however, makes a large enough angle with the normal to the surface, all the light is being reflected.



There still is one detail missing in this explanation to be true, and that is that the light must be incident to a material with a lower index of refraction (this is indeed the case in the example stated). The index of refraction is a property of the material, determined by the speed of light in this material. In an optic fibre, the core of glass is surrounded by the cladding (a coating with a lower index of refraction), which makes sure total internal refraction occurs. The cladding itself is coated with a material that prevents physical damage and protects it from moisture, the buffer. This buffer may be further surrounded with a jacket that improves the strength properties of the fibre.



The situation we just described doesn't really exist. It's not possible to produce a core without any impurities, these impurities cause loss of information. This loss of signal, however, is less than that in copper wiring. Another advantage is that optical fibres are very thin: 10-70 microns. Because optical fibers are thinner than copper wires, more fibers can be bundled into a given-diameter cable than copper wires and more information can be transported. Since optical fibres use light signals, no interference occurs between adjacent fibres and the signal stays clean and clear. All this makes optical fibres the wiring of the future!

Faraday cage

The Faraday cage is named after Michael Faraday, who invented it in 1836. A Faraday cage is a box made of conducting material (e.g. metals) and is used to block out external electric fields. The existing external field exerts a force on the charges of the conducting material. This force produces a redistribution of the charges in the conducting material, causing a certain separation of positive and negative charges. This unbalanced situation results in a second electric field that is directed in the opposite direction of the external electric field, and has the same magnitude of the external electric field. In the box both electric fields cancel eachother (cfr. image).


A well-known example of a Faraday cage is an automobile. If the car is struck by lightning, the biggest part of the charge remains on the metal skin of the car, and little or no electric field is produced inside the car. The phenomenon of blocking external electric fields is also used and needed when no electric interference is wanted (ex. when sensitive electric equipment is used).

Perpetual Motion

A general definiton of perpetual motion machines is that these machines produce more energy or work than the amount of energy they consume. A perpetual motion machine could extract the needed energy for its motion from the energy it produced, this explains why it's called perpetual motion. Such a machine would mean the end of all energy problems. However, it's physically impossible to create a machine like that. There are two fundamental laws that counter the existence of these kind of machines: the first and second law of thermodynamics. The first law of thermodynamics expresses the conservation of energy. Energy can transform from one form into another, but it can't be destroyed or created. The second law of thermodynamics states that no machine can be more efficient than a Carnot heat engine. A heating engine extracts heat from a hot reservoir at temperature Th and gives this heat to a cold reservoir at temperature Tc, with the purpose of heating the cold reservoir. A cooling engine works in the same way, but with the purpose of cooling the hot reservoir. The efficiency of a Carnot heat engine is then given by:

e = 1-Tc/Th

The temperatures Tc and Th are expressed in Kelvin. This means that Tc/Th is always positive, and e always less than or equal to 1. The efficiency e can be equal to 1 if Tc is equal to zero. There are however theoretical reasons for believing that an absolute temperature of 0 Kelvin can't be attained experimentally (e.g. the third law of thermodynamics). We conclude that e is always less than 1, and a machine can never produce as much energy as it consumes and perpetual motion machines are not very likely to exist.

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