Summary:
Introduction + + + Reflection
Refraction - Diffraction
Huygens
waves
The wave theory
A computerized pool!
+ Spherical aberration and Telescope
+ Dispersion and Newtonian
+ + luminiferous ether
Birefringence and Polarization Interference
+ + The rainbow
theories that we seen in previous chapters, relating to reflection and refraction , implicitly argued that light was made of corpuscles (corpuscular theory ) which normally move in a straight line. With the particles could explain the following phenomena:
- Reflection: as a ball bounces on the floor, the particles of light bounce off a perfectly elastic collision (no loss of speed) against reflective surfaces;
- Refraction Here theories are more complicated. Snell, especially Fermat, explain the laws of refraction due to a change in the speed of light passing through various means, but no one can explain convincingly how do the particles, in addition to losing speed (eg passing from air to glass in a lens), to regain (from glass's in the air).
many problems remain open: for example, where do the particles that are not reflected or passing through a transparent medium? If you do not go nowhere, accumulate somewhere?
Diffraction
The 1665 is a turning point: it is published (posthumously) the Treaty "De Lumine" Francesco Maria Grimaldi (Jesuit interest in physics and astronomy, 1618-1663 ) in which he describes for the first time some of the light phenomena never seen before.
Grimaldi notes that the sunlight passed through a small opening and projected onto a white screen at a great distance, casts a halo much larger than would be expected due to simple linear propagation of light, in addition, the edge of the halo is affected by fringes in assorted colors:
According to Grimaldi, it is as if the wave "break" (hence the name of the phenomenon, coined by him, from the Latin "diffractus" past participle of de-feather) and ricomponesse, scattering, beyond the obstacle.
Grimaldi studies go far beyond that certainly too well, for its time. In fact observed for the first time the phenomena of dispersion and interferenza (di cui parleremo prossimamente)... giungendo alla conclusione che la luce potrebbe essere qualcosa di immateriale: l'ipotesi rivoluzionaria di Grimaldi è che la luce potrebbe essere trasmessa per mezzo di onde . ▲
Huygens
Le intuizioni del Grimaldi furono prese in considerazione da un altro dei personaggi incredibili che costellano la storia dell'ottica: Christiaan Huygens.
Matematico, astronomo e fisico olandese, 1629-1695), ha dato contributi fondamentali in tutti i campi which has been busy:
- perfected the clock pendulum Galileo
- He invented the balance spring, getting the first watch at sea transportable;
- Adopting a spiral spring patented the first pocket watch;
- He discovered the rings of Saturn and its largest moon, Titan;
- studying the dynamics of rigid bodies (eg moment of inertia, centrifugal force) ...
esteemed scientists of his time, Newton called him "Summus Hugenius" !
Influenced by written Grimaldi, Huygens began to study whether the properties of light can really be explained by the propagation of waves. ▲
waves
But before going into the studios of Huygens on light waves, so I think it is better to deal with a little 'more manageable and easily understandable: the waves that propagate in the water. I wanted to show in order of water photographed live, but not available:
- a large enough pool;
- infinity of time it takes for the water after each test to come back in perfect stato di riposo;
— di un'attrezzatura fotografica adatta, sospesa a vari metri d'altezza sopra il pelo dell'acqua...
... mi sono arrangiato scrivendo un programma che simula il comportamento della superficie dell'acqua (maggiori dettagli, per chi fosse interessato, si trovano in fondo a questa spiegazione ).
Vediamo cosa succede lasciando cadere un sasso virtuale in questa piscina, anch'essa virtuale:
Le onde si propagano a velocità costante dal punto in cui viene lanciato il sasso e, come c'era da aspettarsi, le onde che si formano sono perfettamente circolari.
Se ci allontaniamo a sufficienza dal punto that has thrown the stone, the wave front becomes straight with the motion of the wave perpendicular to the wave front itself:
( The plane waves, or straight, are easier to studies, and in fact the light from the sun or the stars are generated at distances so large that they can safely be regarded as parallel rays).
Now let's see what happens when a packet of plane waves invests the pool:
When the incident waves (red arrow) reach the bottom border (in gray), and in some way "bounce" able after a while 'to regenerate the reflected waves (green arrow) perfectly symmetrical to those incidents, because the angles α and β are equal. The generation of reflected waves is perfectly, even though in the area close to the edge you see the strange triangular plot: This is an area where the incident and reflected waves interact with each other, adding or canceling each other depending on where you look at that, despite this apparent chaos, the reflected waves, as already mentioned, are regenerated perfectly.
New question. The waves can be created, in addition to the phenomenon of reflection, the refraction? To do the test should be a pool containing two different liquids: one part water, the other in another liquid, say oil, in which the waves propagate at different speeds, and all with a sharp dividing line: not exactly an easy thing to achieve in reality! But with my virtual pool it is really easy!
In the animation below you can see the liquid of two colors: the water is to the left and right, there is a green oil in which the waves travel at half speed over water. Let's see what happens when a packet of plane waves is directed diagonally towards the line of separation of two liquids:
Initially the wave is moving only in the water (Red arrow) when it reaches the line between the two liquids, the waves begin to propagate even in the oil (blue arrow), not in the same direction, but even according to the angles α and β which are derived from Snell's law we saw in the previous episode! But ...
... continuing with the animation, we see that the incident wave also generates a reflected wave (green arrow). In fact, the refraction is not something that ever happens by itself: a phenomenon of refraction is always associated with a phenomenon of reflection (the reverse is not always true). This is why watching a shop window you see both exhibits the reflection of people walking on the road!
Now is the time to verify if the diffraction discovered by Grimaldi could be interpreted as a wave phenomenon:
left us in the open sea, where they move to the right, the plane waves. Which shall be crashing into a dam where there is a small opening: my virtual pool shows that the waves do not continue in a straight line, but "scattered" in all directions, just as perceived by Grimaldi.
Note: the phenomenon of diffraction we all experience: in fact the sound, which propagates through the waves, allows two people to talk to one room to another, but to do that the sound must be able to able to "turn the corner." ▲
The wave theory
Having seen all these beautiful things, we can go back to Huygens and his studies on the wave nature of light. For his model of propagation of light, Huygens was inspired by the propagation properties of mechanical impulses that had already analyzed the occasion of his studies on the pendulum. The hypothesis is that all space is permeated by some means incorporeal, the so-called "ether luminiferous "
When a" ball "of this ether moves for any reason it immediately transmits its movement to adjacent beads, which in turn forward it to adjacent beads, and so on. This basic mechanism, described in his "Traité de la lumière" 1690, is the basis of "Huygens principle":
Each element of a wavefront can be considered as a secondary source of spherical waves [...] The perturbation produced by a point in space you can always get as a superposition of all spherical secondary waves that reach that point.
See you then how these "spherical waves" begin with the phenomenon of reflection (the animation below shows the simulations have already seen above):
The plane wave shown in red, from the left , progressively meets the wall of the pool below. In each of these points of contact (in the animation will show only a few) the incident wave disappears and is replaced by a source of spherical waves (indicated by red dots) that propagate at the same speed at which the move ' incident wave. The envelope of the fronts of spherical waves, "reconstructs" perfectly reflected wave.
Now, the refraction
The incident wave is still moving from left to right, to be replaced by sources of spherical waves as they meet the line of separation between the two liquids. The speed of propagation which have different here is that the balls (or rather, hemispheres) will expand at different speeds on the right and left, building is the reflected wave (blue, left) that the refracted (green, right) .
The diffraction can be explained through the principle of Huygens:
The plane wave coming from the left is absorbed by the dam and replaced by the usual sources of spherical waves riding the opening here on the right that the wavefront propagates in all directions.
studying the propagation of waves, Huygens is able to obtain all the already known laws of optics, such as Snell's law on the refraction and reflection-refraction phenomenon twice, plus also explains other phenomena such as diffraction and interference : really a remarkable achievement! Of course, the wave theory leaves open some issues such as the definition of the medium in which the waves propagate, the 'luminiferous ether that will be searched for another couple of centuries.
Unfortunately, his theories are not taken into account much by his contemporaries due to the influence of two giants as Descartes and Newton, both strong supporters of the theory Corpulscolare of light in the early nineteenth century, however receive his deserved recognition, with consequences quite extraordinary! ▲
A computerized pool!
My virtual pool consists of several elements arranged in a checkered pattern: imagine each item placed on top of a different box with foil into squares. Each element can be move freely about its vertical position:
Each element is linked to those adjacent to elastic, so the vertical movement of an element generates forces that act on those that are next:
Above items blue are held still while the green ones are dragged from the red one. For each cycle of calculation, taking into account the height and speed of all the elements, the computer
- calculates the forces acting on each element due to differences in height compared to adjacent, and thus calculate a acceleration value for that item;
- changes the speed of each component calculated according to the acceleration;
- change the height of each element based on its speed.
The fact that each element is equipped with mass means that it can not instantly change its position with respect to the applied force (principle of inertia). This delay is most obvious when the elements become:
In simulations I used "pools" composed of several hundred elements per side ... Think that makes the computer work, which still has not learned to protest.
What is really fascinating is that, with a model so simple (in fact the behavior of each element of the pool is simple: the program that simulates requires very few instructions) may give rise to such complex phenomena! ▲
Next Chapter: Telescope and spherical aberration
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