Refraction, the Critical Angle and Total Internal Reflection.


The idea behind optical transmission is to take a ray of light and send it down a waveguide to a receiver. The wave guide is of course an optical fibre cable.

The greatest efficiency when transmitting by optical fiber comes when total internal reflection occurs. This condition arises from a number of factors to do with refraction, which we shall explore in turn.

The first is refraction itself. This is a term that pops up everywhere we have visible light; it is the reason why a straight stick looks bent  when  half is placed in water and half in air. It is the bending of light.

Contrary to popular belief light speed C is not constant through all media. In free space the speed is 299,792,458 metres per second, in air it is slightly slower, and in other materials slower still. It can be seen then that if it passes a boundary between  two materials then light will either speed up or slow down as it passes from one to the other. The speed of light passing through a material can be measured and given a comparative value to the speed of light in free space, C. This is done by dividing C by the speed in the material, and the resulting dividend is known as the refractive index.

As the velocity changes in magnitude so does it in direction, due to the wavefront travelling further on one side than the other. The drawing below indicates this in graphic form:


On the left you can see a light ray passing  through the boundary between materials with different refractive indices. The direction of the ray changes as it passes from one to the other; in this case it is coming from the bottom to the top.  The angles of  incidence and refraction are measured from the normal, which is an imaginary line at 90 degrees to the boundary.


In the material with the lower refractive index, the wave moves at a higher speed, in the higher refractive index a lower speed.

Unlike reflection the angle of refraction is not necessarily equal to that of  incidence, and in fact there is a mathematical correlation between the two. We look at this on the next page.

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