Why the refractive index of inner tube is greater than the outer tube in optical fiber?

Answer 1

Back in the 1870s, British scientist John Tyndall performed a demonstration in which he showed light guiding by means of a curved stream of water flowing from an illuminated tank.

Between 1900 and 1930 numerous experiments followed Tyndall's demonstration. It was discovered that bent thin glass rods not only could transmit light, but using a bundle of glass rods (or optical fibers, as they were called later) complete images could be carried as well.

What type of phenomenon was involved in these sensational findings? Well, it was just reflection, total internal reflection, as the walls of the thin fiber acted like mirrors in which the incident light bounced back and forth.

You have many examples of total internal reflection in your everyday life. Just look for down asphalt road on a hot, sunny day, and you should see reflection from the road surface, as if the road had become a mirror. Well, it has due to the same property that causes internal reflection: if light hits at a low enough angle, it can't penetrate the surface, and just bounces off. Weird, hey?

This reflection, governed by Snell's law, is made possible due to the differences between the refractive index of the glass and the air, the latter having the lowest value.

However, the usefulness of light guidance would not be completely appreciated until 1950, when many scientists began to think in its potential applications. They were smart enough to understand that such applications ranged from medicine, enabling the visualization of inaccessible regions of the human body, to communication networks, in replace of metal wires.

But there were a number of drawbacks that arose in those first applications: light couldn't be carried too far, and the transmitted images came pretty poor and distorted.

That was because the fibers had no coating at all surrounding them to increase the difference in refractive index. For that reason, if such fibers got wet, for example, the glass-air interface didn't remain fixed and the way the light reflected inside was changed.

Only in the mid 50s, Dutch Abraham van Heel designed a covering layer surrounding the fiber with another cylinder of glass, of lower refractive index than that in the middle. This way, the total reflection was not affected by water, dirt or other contamination. Finally, a third layer was added, in order to protect the glass fiber from damage, and to make it easier to handle.