When light passes from one material to another the direction of the light usually changes. This is because of a property called the refractive index of a material, labelled by
The light changes direction according to Snell's Law, given above. This is illustrated in the diagram for light passing out of water into air.
The refractive index of air is taken to be 1. If the refractive index of water is 1.33, then if we can measure
we can find
by calculation. Suppose we measure
to be 40 o .
o
Total Internal Reflection
From Snell's Law,
there is a certain vale of
called the critical angle, for which
This value is labelled
and is illustrated below.
For
total internal reflection occurs. No light passes through the interface between the two materials.
For
the angle of refraction is 90 o . All the light passes along the interface between the two materials.
For
the light is refracted as it passes from the first material into the second. This is shown in the first diagram above.
Fibre Optic Cable
Light strikes the interface between the fibre optic thread and the cladding repeatedly but the angle of incidence is always less than the critcal angle so total internal reflaction repeatedly takes place and the light can pass for long distances along the cable.