Sommario:
+ Introduzione
+ Riflessione
+ Rifrazione
+ Diffrazione
+ Cannocchiale e Aberrazione sferica
+ Dispersione e Telesopio di Newton
+ Etere Luminifero
– Birifrangenza e Polarizzazione
Lo Spato d'Islanda
Newton e Huygens
La Polarizzazione della Luce
Onde Longitudinali e Trasversali
Étienne-Louis Malus
Experiments homemade
Interference
+ + The rainbow
The Spato d ' Iceland
In previous chapters we have left Newton and Huygens to bicker on the different interpretations of the light rays. Corpuscles were, as Newton claimed, or waves, according to the thought of Huygens?
Year 1669: twist. The Danish medical Rasmus Bartholin (1625-1698) observed a strange mineral: a type of calcite, also called Spato slide or Iceland spar (one of the photo above). Bartholin note that objects viewed through this material appear split, as if this calcite was able to simultaneously produce two different refractive indices, hence the name of this strange phenomenon:
In this picture, the crystal is supported on a white sheet bearing the printed word CALCITE with simple characters and the fact that the word is double, staggered vertically, is the result of a truly exceptional behavior of this material (and a few others) is absolutely natural: birefringence.
Let's see what happens to a beam of light (shown in purple, from da sinistra) che attraversa un cristallo di Spato d'Islanda, perfettamente lucidato e con le facce parallele fra loro.
Secondo la legge di rifrazione, un raggio di luce che arrivi sul cristallo in direzione perfettamente perpendicolare alla sua superficie non deve deviare, né la cosa deve accadere all'uscita del cristallo stesso: nell'immagine qui sopra tale raggio è indicato in rosso, e viene chiamato "raggio ordinario". Il cristallo però esibisce un fenomeno aggiuntivo: all'interno del cristallo, una parte della luce (il cosiddetto "raggio straordinario") segue un percorso diverso, in modo da fuoriuscire da un punto diverso. All'uscita dal cristallo i due raggi proseguono paralleli fra loro... ma cosa mai può essere accaduto all'interno del cristallo affinché il raggio in entrata si sia diviso in due? ▲
Newton e Huygens
Immediatamente iniziano i tentavi di spiegare il fenomeno: per Newton la causa sta nei suoi corpuscoli, che potrebbero avere qualche asimmetria nella loro struttura. Huygens dal canto suo è convinto di riuscire a spiegare il fenomeno grazie alle sue onde longitudinali, ma un bel giorno...
... proprio Huygens esegue un esperimento che lo farà capitolare. Egli prende due cristalli e li allinea in modo che ciascuno di essi faccia deviare il raggio straordinario esattamente nella stessa direzione. Poi fa passare un raggio di luce attraverso i due cristalli... cosa succederà adesso?
Huygens è convinto che ciascuno dei due raggi uscenti dal primo cristallo abbia la stessa natura del raggio di luce entrante, ovvero un fascio di onde longitudinali. Si aspetta quindi che nel passare dal secondo cristallo i due raggi diventino quattro, o qualcosa del genere... ma siccome il risultato dell'esperimento è confuso, per capire meglio cosa sta succedendo oscura uno dei raggi per volta.
Huygens saw with surprise that, as shown in the figure, the extraordinary ray does not split completely but suffered a second extraordinary refraction, and if you let pass only the ordinary ray, this continues in a straight line (according to the normal law of refraction) through both crystals.
's astonishment stems from the fact that Huygens waves evidently "preserve" the memory of the journey already undertaken, in short, have some attribute that enables them to behave differently if they have already passed through a crystal, or they have not yet done ... this attribute and longitudinal waves do not have it their own. Newton says the fact that light "has sides "... thing, but not a complete explanation of the phenomena, in fact even Newton has very clear ideas about it.
remember that in this first experiment Huygens oriented crystals in the same direction "optical". By rotating the crystals to each other, he gets a very complex series of results which at one point declare
not be able to identify the causes. But for this reason not to desist from describing the purpose of giving others the opportunity to investigate them.
To move forward in our understanding of these phenomena will have to wait more than a century in this period la scena sarà dominata dalle teorie corpuscolari di Newton (più per l'importanza del personaggio che per ragioni scientifiche) in quanto non era stata ancora trovata nessuna prova decisiva. ▲
La Polarizzazione della Luce
Nel 1808 viene fatta, in modo più o meno casuale, un'importante scoperta: l'ufficiale francese (ma anche ingegnere, fisico, e matematico, 1775-1812) Étienne-Louis Malus sta "giocando" con un cristallo di calcite, quando si accorge che uno dei raggi... è sparito! Girando il cristallo, vede attenuarsi this radius and "light up" the other goes off until the first and second reaches its maximum intensity.
At first I think of a crystal "bad", but then one suspects that the phenomenon could depend on the light passing through it: it is not using the direct sunlight, but a light beam reflected from a window . Then repeat the experiments with direct sunlight: the crystal in this case is operating normally, with the two beams still visible, including the extraordinary "revolving" normally together with the crystal.
Malus had discovered a new phenomenon: the polarization of light because of its reflection. Malus was a staunch supporter of the wave theory of Huygens. In making his experiments, however, accumulates more and more clues that suggest that it is not longitudinal waves, as Huygens believed, but rather cross. But what did they ever different these two types of waves? ▲
longitudinal and transverse waves
to imagine a rope swing in the vertical plane as those who use children to jump. If the rope passes through two boards of a fence, the wave propagates without any problems:
as if the rope passes through a horizontal slit, the wave can not get over the obstacle
The rope that swings as shown in these animations move in a transverse wave motion ; longitudinal waves in this phenomenon instead of "arrest" does not occur:
Let's see what happens if the waves are neither horizontal nor vertical, or if they are neither aligned nor perpendicular to the slit that serves as obstacle
Each wave can always be decomposed into two "projections" orthogonal. In the face of the left is in breach of the plane on which direction the light wave propagates, and this is projected to the horizontal plane (blue) and vertical (red). On the right we see the development of the original wave (purple) and its decomposition (red and blue), the sum of these two waves results in exactly the original broadcast, so the two representations of this beam of light are absolutely equivalent . In the last section of the right is only transiting the vertical component of the wave, as if at that point had been applied the "fence" vertical (ie, a properly oriented polarizing filter).
Obviously the Sun sends us rays of light, so to speak, "bulk", that is oriented at any orientation transverse oscillation. To simplify the calculations can be broken down into its components within each vertical and horizontal
left, in purple, you see a number of variously oriented view, in the central part of their displacement, the right result total of the components of horizontal and vertical incident waves. ▲
Étienne-Louis Malus
Let us now return to work Malus. Following its discovery began to study the effects of refraction and reflection of polarized light, or composed only of light waves that lie on one plane at a time. Snell's laws describing the angles of reflection and refraction (ordinary) remain valid, irrespective of the angle of polarization of the incident light. What changes is the intensity, the light impinging on a transparent material is partly reflected and partly refracted, but these proportions vary with the polarization of light.
In 1810 Malus publishes its findings, describing the behavior of reflection and refraction on different materials taking into account this new concept of polarized waves. His results are correct with regard to water, but not for the glass, but through no fault of his own: at the time the glass was not yet produced the necessary characteristics of optical precision (in particular, many layers of glass' s age exhibited a different coefficient of refraction on the surface than within them). The witness will
other eminent personalities, which I'll discuss in the next episode. Remains open the fundamental question: What makes the light? Particles or waves? The scientists were still convinced that they can explain everything according to the corpuscles, since it was already clear that it was impossible to do with longitudinal waves. On the other hand, the transverse waves posed a number of theoretical problems, since the medium in which they propagate. As mentioned in the previous episode, the "luminiferous ether", to enable the transverse waves travel at the speed of light, should have been more hard steel, but this light did not reveal its presence in any kind of scientific experiment a part of those optical. ▲
Experiments homemade
required:
- a crystal of Iceland spar. It is located in stores of minerals, costs around ten euro.
- a polarizing filter for the camera.
- a laser pointer. Do not sell it more as a toy, but it can be found in office supply stores, if they find the price of a ten euro.
Warning: never point the laser directly into the eyes, nor their or others!
The first experiment is to place the crystal of calcite on a printed, resulting in a doubling of effect similar to that which I mentioned above. Superimposing the crystal polarizing filter, and rotating, we see an effect like this:
The light that illuminates the message passes through the crystal divides into two beams with different polarizations, orthogonal to each other. The filter allows you to see one at a time, when the alignment between the plane of polarization of a beam and the filter are perfectly coincident, or both beams when the filter is rotated in an intermediate position between the angles of polarization of the two light beams.
Second experiment: instead of taking up the ambient light, we use a laser beam in a dark room. For the following photos I arranged things this way:
The ray of light generated by the laser pointer is passed through the polarizing filter B and then, through the mask is equipped with a small circular hole (the output of the laser beam is not exactly my point, and needs to be "cut"), reaches the crystal projected on the screen D to E . (thank the aldoaldoz for allowing me to borrow the use of clothes pegs, which are essential to keep the laser pointer on and to keep in place all the objects shown in the photograph).
Here is the result of the experiment:
Bottom three photos of the light projected on the screen, made with a different orientation of the polarizing filter at top, the crystal fotografato durante l'esperimento: si vede qualche alone di luce rossa, dovuto a fenomeni di diffrazione che si manifestano sulle superfici del cristallo. ▲
Prossimo capitolo: Interferenza
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