City of Light: The Story of Fiber Optics

GUIDING LIGHT AND LUMINOUS FOUNTAINS 13
ladon was still an active scientist. He was famous for inventing compressed
air, which powered machines deep underground and delivered fresh air to
miners, making their job faster and safer. Yet he felt cheated because Italian
engineers won the contract to build the first long railroad tunnel through the
Alps using his idea.
The fountains that lit up the City of Light in 1889 used another of Colladon’s
inventions. Forty-eight years earlier, he had shone a beam of light
along a jet of water, to entertain lecture halls in Geneva and London with an
optical trick called total internal reflection (see box, pp. 24–26). Now a new
generation of engineers had made his invention a thing of beauty on a much
grander scale, building a tableau of illuminated fountains before the main
entrance to the Universal Exhibition. 3 The public marveled at how electric
lights—still a novelty—illuminated the fountains at night. Some lights played
on the water from above; others shone up from the base of the fountain or
along jets emerging from the sides of sculptures. Trapped within the water,
the light passed along the jets until it emerged sparkling, as it had in Colladon’s
lectures, and as it would in optical fibers.
The Man Who Guided Light
Daniel Colladon was a 38-year-old professor at the University of Geneva when
he first demonstrated light guiding in 1841. 4 He wanted to show fluid flow
through various holes and the breaking up of water jets observed earlier by
French physicist Felix Savart. 5 Colladon’s experiments worked, but at first the
audience couldn’t see the flowing water in his lecture hall, particularly with
the poor lighting available in the mid-nineteenth century.
He solved the problem by collecting sunlight and piping it through a tube
to the lecture table. A lens focused the light through the water tank and
along a jet squirting out a hole in the other side. When the light rays in the
water hit the edge of the jet at a glancing angle, total internal reflection
trapped them in the liquid. They bounced along the curving arc of water until
the jet broke up, as shown in figure 2-1. Instead of traveling in a straight
line, the light followed the curve of the water.
In a dark room, the effect was impressive, ‘‘one of the most beautiful, and
most curious experiments that one can perform in a course on optics,’’ Colladon
wrote. ‘‘If the water is perfectly clear, and the opening of the diaphragm
very [smooth], the stream is scarcely visible, even though a very intense light
circulates inside it. But whenever the stream encounters a solid body that
obstructs it, the light that it contains escapes, and the point of contact becomes
luminous. ...Ifthestream falls from a great height, or if its diameter
is only of some millimeters, it breaks apart into drops in the lower region.
Then only does the liquid give light, and each point of rupture in the stream
casts a bright light.’’ 6
Proud of his new trick, Colladon used it in the public talks to the urban
intelligentsia that were an important sideline for mid-nineteenth-century sci-