Snell's Law, Refraction, Total Internal Reflection and the Critical Angle
When light passes from one material to another the direction ofthe light usually changes. This is because of a property called therefractive index of a material, labelled byThelight changes direction according to Snell's Law, given above. Thisis illustrated in the diagram for light passing out of water intoair.
The refractive index of air is taken to be 1. If the refractiveindex of water is 1.33, then if we can measurewecan findbycalculation. Suppose we measuretobe 40 o .
Total Internal Reflection
From Snell's Law,
Thereis a certain vale ofcalledthe critical angle, for whichThisvalue is labelled andis illustrated below.
Fortotalinternal reflection occurs. No light passes through the interfacebetween the two materials.
Fortheangle of refraction is 90 o . All the light passes along theinterface between the two materials.
Forthelight is refracted as it passes from the first material into thesecond. This is shown in the first diagram above.
Fibre Optic Cable
Light strikes the interface between the fibre optic thread and thecladding repeatedly but the angle of incidence is always less thanthe critcal angle so total internal reflaction repeatedly takes placeand the light can pass for long distances along the cable.