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Light Speed

Jeff Hecht

Jeff Hecht is the author of CITY OF LIGHT: THE STORY OF FIBER OPTICS (Oxford University Press), and UNDERSTANDING FIBER OPTICS (Pearson Prentice-Hall). Trained in electronic engineering at Caltech, he has written about science and technology for over a quarter-century, with particular focus on lasers, fiber optics, and optics. A full-time free-lance writer based in the Boston area, he writes regularly for NEW SCIENTIST and LASER FOCUS WORLD.

I would like to find out more about the mathematics involved in the trapping of light in the stream of water. Is this published?

A: Standard optics books cover the mathematics of light trapping in a jet of water. It is caused by a phenomenon called total internal reflection, which is closely related to the bending of light by a lens, called refraction. Refraction occurs when light passes between two materials with different values of a property called refractive index, which measures the speed of light in a material. Total internal reflection occurs when light in a high-index material like water or glass hits the surface at a glancing angle smaller than specified by the equation


Air has a refractive index of 1.0003 and water has a refractive index of 1.333, so the light has to hit the surface of the water at an angle less than 41.4 degrees.


What was the name of the box the Victorian era physicists used to bend light, and where could I find plans for one?

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It had no special name. It was a tank that held water with a spout on one side for water to drain out, and a clear region on the opposite side for light to illuminate the water. A diagram of one used by Daniel Colladon appears on page 14 of my book, CITY OF LIGHT.


When will fiber optics be available for surgery in the U.S.? I am an RN who works in surgery, and the idea of fiber optics is very impressive. I was shocked!

Optical fibers are now used in "keyhole" surgery, but not all surgeons use them. The usage differs among specialties.


Fiber optics seems to be the way to go, and according to the INNOVATION "Light Speed" show it's cheap to make, so why don't we see fiber optics in every new home? Why are we still using cable and copper phone wire?



Fiber is cheap, and optical transmitters and receivers are only a little more expensive than electronic ones. The problem is that replacing existing coaxial cable and copper telephone wires with fiber would require much costly labor. Fiber is being installed in some new subdivisions and in some rural areas in the United States (see the list compiled by the Fiber to the Home Council at http://www.ftthcouncil.org). Hundreds of thousands of homes in Japan have fiber-optic Internet connections.


In the telegraph days with trans-Atlantic cables, how did they get by without having repeaters on the cables in the ocean?

Telegraphs need only detect dots and dashes, so they can sense very weak signals. Care had to be taken to make the copper cables thick enough to carry current a long distance, and to make sure the pulses did not stretch out so much that the signals took a long time to transmit. William Thompson, later Lord Kelvin, designed the first successful electrical trans-Atlantic telegraph cables to meet these requirements.

Where can I find more information about Canada's fiber optic network?

The Canarie project has information on its CA*net4 at http://www.canarie.ca/canet4/ and a map at http://www.canarie.ca/canet4/connected/canet4_map.html.


When will we be able to accelerate a particle(s) to faster than light speed?

Einstein's Special Theory of Relativity says that nothing can travel faster than the speed of light in a vacuum. So unless Special Relativity is wrong, we can't accelerate any particles faster than light. However, light slows down when it travels through gases or solids. The speed of light in glass fibers is about 2/3rds of the speed of light in vacuum. Electrons can travel faster than that through copper wires, but they can't carry as much information. That is, electrons may get to the other end of a cable faster through copper than light can get to the other end of a cable through glass, but the light delivers more bits per second.


Illustration: "La Fontaine Colladon," published 1884 in the magazine LA NATURE from a paper by Daniel Colladon.


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