Sommario:
+ Introduzione
+ Riflessione
+ Rifrazione
+ Diffrazione
+ Cannocchiale e Aberrazione sferica
+ Dispersione e Telesopio di Newton
– Etere Luminifero
Il vuoto
Il barometro di Torricelli
Otto von Guericke
Propagazione delle onde
Onde Longitudinali
transverse waves
Light: waves or particles?
Vacuum Physics in
+ Birefringence and Polarization Interference
+ + The rainbow
Vacuum
The void is something that has intrigued the "natural philosophers" from the remotest antiquity ... and is not that scientists now know it all all all right! (See below for some additional considerations on the void, even if a bit 'off topic).
The characters most significant ancient Greek Democritus is, just what I had already imagined the atoms. Well, for him they were moving freely in vacuum , moving in uniform rectilinear motion until it went crashing into other atoms (with that anticipated by almost two thousand years the modern principle of inertia).
Unfortunately the theory of Democritus was eclipsed by that of Aristotle sacred monster. Whereby the motion of bodies was not due to its inertia, or as we say today its momentum, but rather the cause of the movement of bodies was in the "middle" in which they moved: a bullet, according to Aristotle, once thrown, would continue because in the motion moved by the air continually scramble to fill the void left by the bullet as it passes.
According to Aristotle, moreover, the speed of the bodies was inversely proportional to the density of the medium through, for example faster in air than in water. Conclusion: the vacuum, which corresponds to a density of course nothing would mean infinite speed, which was impossible. Hence the Aristotelian conviction of the impossibility of the void: "Nature abhorret Vacuum &!"
The topic was also dealt with the vacuum in the Arab world, two of the most famous of the eleventh and twelfth centuries century: Avicenna, who somehow took up the ideas of Democritus, and Averroes who, having written numerous commentaries on the works of Aristotle, it follows more or less positions. ▲
The barometer Torricelli
As you know the ideas of Aristotle pervaded the history of science until the sixteenth century. Even Galileo had trouble admitting the vacuum, while Descartes, again in 1644 in his Principia Philosophiae argued that there is no vacuum. But in that same year, Evangelista Torricelli, assistant to Galileo, the famous experiment where he invented the barometer:
Torricelli filled with mercury a glass tube about one meter long and sealed in a ' ends, then reversed so that the mouth is to rinse in a bowl containing the same material. The result is known: the level of mercury in the tube down to bring into balance the forces due to atmospheric pressure (A) and the weight of mercury (Hg). The big news was ... the volume of the tube at the top, no longer filled with mercury, what we could be there? Absolutely nothing ... behold, there was created a vacuum! ▲
Otto von Guericke's experiment
Torricelli made a tour of Europe in a very short time, because it was relatively easy to replicate an experience . And here comes another remarkable person, Otto von Guericke (1602-1686): he was a politician, lawyer and German scientist, who in 1650 invented the first pump capable of creating a vacuum (like a bicycle pump, but the reverse operation).
Von Guericke quickly realized the enormous forces caused by atmospheric pressure, and found a very dramatic way to demonstrate:
He in fact build two hemispheres in bronze, joined with a gasket and produced a vacuum within them using the pump he had invented. Arranged eight pairs of horses, four on each side, it was not possible to separate the hemispheres, but then sent back to the air inside, the two hemispheres separated by himself!
Von Guericke, in addition to implementing this and other experiments, investigated the properties of the vacuum: for example, discovered that a candle goes out into the void, and that the animals can not survive. But the crucial discovery from the point of view of optics was:
an experiment similar to this picture, he realized that light could pass through the gap, but no sound, noise generated at ' inside a container in which it was created outside the gap do not propagate. ▲
Wave propagation
course now would go a bit 'too off topic se parlassi del suono... comunque qualche accenno lo considero necessario.
Gli studi iniziano nell'antica Grecia, già a partire da Pitagora che studia i rapporti numerici fra le altezze delle note: Pitagora metterà poi questi rapporti in relazione con le grandezze delle sfere celesti, da cui la famosa "armonia delle sfere"!
Il primo moderno ad occuparsene fu, manco a dirlo... Galileo Galilei, forse anche perché suo padre Vincenzo era un compositore e teorico musicale. Fra le altre cose, Galileo studiò le corde vibranti e riconobbe il legame fra altezza del suono e la sua frequenza. Nello stesso periodo, proprio quel Marin Mersenne che abbiamo incontrato al tempo della disputa between Fermat and Descartes about the refraction of light, measures the speed of sound in air. And the Von Guericke? After discovering that the sound propagates in a vacuum, but discovered that he did very well in both liquid and solids.
With regard to the propagation of sound, there was almost unanimous belief that they were waves which propagate forever in half, whatever it is ... and his discovery that the sound does not propagate in vacuum it was further proof. ▲
Longitudinal waves
Here we are talking waves ... but what exactly are they? There are only one type?
begin with the simplest case: the propagation of sound in air. Let us assume that the air is made by a series of "bubbles" that you touch with each other, causing a bubble in the first movement, this movement spreads to those nearby:
This is outline what happens in the propagation of sound in air, in which the waves propagate in a "longitudinal", ie the disturbance propagates in the same direction you move the bubbles themselves.
About our row of bubbles, however, we can generate a altro tipo di movimento:
Qui il movimento viene generato in direzione perpendicolare alla linea di propagazione che vogliamo studiare. La bassa viscosità dell'aria fa sì che questo movimento venga propagato di pochissimo alle bolle adiacenti, di fatto impedendo a questo tipo di perturbazione di propagarsi. ▲
Onde trasversali
Ma se invece dell'aria usiamo una barra metallica? Ebbene, tirando una martellata in senso longitudinale, la vibrazione si propaga più o meno come nell'aria. Se il colpo viene dato in senso trasversale alla barra metallica, si ottiene invece un effetto molto più significativo che nell'aria:
La martellata data in questo modo genera un'onda "trasversale", cioè un'onda in cui la perturbazione si propaga in modo perpendicolare rispetto al movimento delle singole parti della barra di metallo. Detto in altre parole, i singoli pezzetti della barra oscillano sulla loro posizione verticale, mentre la perturbazione trasla da sinistra a destra, in orizzontale.
Nei liquidi, e ancor più nei solidi, le onde longitudinali e trasversali coesistono senza problemi. E questo è proprio ciò che accade anche nei terremoti! ▲
Light: waves or particles?
We talked about empty, the waves do not propagate in vacuum, and that move in a longitudinal or transverse, depending on the medium in which they propagate ... Now we have to go back to talking of Light.
At this point the assumptions about the nature of light are three:
- The light consists of particles (Newton), and this justifies the propagation in a vacuum;
- The light is made up of longitudinal waves ( Huygens), which explains some phenomena in this way as reflection and refraction. These waves propagate in a "luminiferous ether" that permeates the entire universe: soft enough to be felt in other ways, but with other features tailored to justify the transmission of light;
- The light consists of transverse waves. For the moment no one sees it as a plausible hypothesis, because this type of light waves require a "solid" to justify the speed of propagation of light, which had not yet been measured but which could still be very high, ether it would take a harder than steel, and at the same time "Thin", and will not be warned. ▲
The physics of the vacuum
This section is a bit 'off topic, but I have to justify my impertinent remark, that physicists would not have yet figured it all around the void. Below I describe some facts, no matter how apparently bizarre and difficult to digest, they were really occurred!
In no corner of the universe can be said that there is a totally empty space. If it did not contain even an atom, a certain region space would be traversed by light photons, neutrinos, gravitational fields, and who knows what else.
on the assumption he will make a vacuum chamber totally devoid of matter inside. The walls of the vacuum chamber, however, emit light in the form of infrared rays, then we can say that the vacuum has a particular temperature, or a certain amount of energy we can not say I got a perfect vacuum. Let
then bring the vacuum chamber at a temperature of absolute zero: the photons in these conditions are no longer generated, then we would be tempted to say that we finally got a completely empty space, lacking both di particelle che di energia. Ma a causa del principio di indeterminazione di Heisenberg, l'energia non può non essere soggetta a fluttuazioni: ecco quindi un ribollire di coppie di particelle virtuali, che nascono e si annichiliscono in continuazione.
Insomma il vuoto viene interpretato dalla meccanica quantistica come un equilibrio dinamico di particelle di materia e di antimateria in continua generazione e annichilazione. Ciascuna di queste particelle va interpretata secondo la dualità onda-particella... per cui ecco, proprio a proposito di onde, è giunto il momento di descrivere un fatto teorico (verificato dalle esperienze di laboratorio) veramente straordinario!
In uno spazio vuoto di dimensioni Finally, any particle, real or virtual, can vibrate at any wavelength. But in a confined space, as in a vacuum chamber, the wave-particle can vibrate only at wavelengths that are multiple and submultiple of the distance between the walls.
then imagine the following situation: inside a vacuum chamber, we put a pair of conductive plates, very close to each other. The waves can be generated between the plates (and then in a tight space) are subject to greater constraints than those that are generated in the rest of the vacuum chamber, in other words, there are many more lengths d ' wave plates allowed outside than within them:
This means that the energy density outside the plates is greater than the density within them, which translates into a measurable difference pressure: this phenomenon has the name " Casimir effect ", named after the Dutch physicist Hendrik Casimir (1909-2000).
In summary, if in a vacuum chamber, put two plates sufficiently close, you can measure a difference in pressure between the inside and the outside of the plates themselves. But how can the empty generate a pressure difference between an "inside" and an "outside" that are in communication with each other? Mysteries ... of quantum mechanics!
Casimir effect have been made many experimental measurements, but given the practical difficulties (due to the distance between the plates, which must be in the order of one thousandth of a millimeter, and the low value of the forces) to final confirmation there was before 1997: the Casimir therefore had time to see his theory confirmed! ▲
Next Chapter: Birefringence and Polarization
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