Designing
Sandrine Ferri, Senior Lecturer, Laboratory of Physics of Ionic and Molecular Interactions (PIIM)
Classes from the 3rd grade onwards
Did you know that light can be guided? That she can bend, reveal herself? That it is used to carry information? During this workshop, the researcher will work with the students to understand how an optical fibre works and to introduce optical telecommunications.
How to bend a light beam, optical fiber principle
There are several types of optical fibers. In jumping index fiber, the refractive index changes abruptly between the core and the cladding. In graded index fibre, this change in index is much more gradual. When a light beam enters an optical fiber at one end with an adequate angle, it undergoes multiple internal total reflections. This beam then spreads to the other end of the optical fibre without loss, following a zigzag path.
The curvature of a light beam can be reproduced in a "laboratory", either by passing a LASER beam through a tank filled with water; the beam entering the tank at a slight angle (image A). Either by passing a LASER beam through a medium of variable index: by adding coarse salt to the water, the light beam is deflected down the tank; by adding alcohol to the water, the light beam is deflected upwards. On the pictures, the length of the tank is about 40 cm.
Handling of optical fibres
An optical fibre is a very thin glass or plastic wire that has the property of conducting light and is used in terrestrial and oceanic data transmissions.
The so-called "UFO" decorative lamps, fashionable in the 1970s, use plastic optical fibres. The light from the base is either produced by white or coloured LEDs or by a white bulb accompanied by a carousel of coloured filters.
The optical fountain
In an experiment on light, Colladon in 1841 showed that a beam of light can be guided by a water jet. John Tyndall, in 1870, explained this phenomenon by the multiple reflections on the surface.
An intense light beam is introduced along the axis of a water jet. Due to the multiple reflections, the beam cannot leave the jet and follows its curvature. The water jet is expected to remain completely black as long as it has only a small change (droplet breakage). The light can then escape and the water jet lights up. This effect is also known as an "optical fountain".