1.1. To understand the concept of To understand the concept of TTotal otal IInternal nternal RReflection eflection (T.I.R.)(T.I.R.)

2.2. To be able to apply TIR to applications such as To be able to apply TIR to applications such as fibre optics and gem stonesfibre optics and gem stones

Book Reference : Pages 193-195Book Reference : Pages 193-195

Total Internal Total Internal ReflectionReflection is an example of is an example of refractionrefraction (note the difference in naming!) (note the difference in naming!)

1.1. We’ve seen that when light travels from a We’ve seen that when light travels from a more to a less optically dense material the more to a less optically dense material the light is refracted away from the normal light is refracted away from the normal (e.g. glass into air)(e.g. glass into air)

2.2. At a certain incidence angle the light is At a certain incidence angle the light is refracted along the boundary between the refracted along the boundary between the two materialstwo materials

i i

i

More Dense

Away from NormalLess Dense Refract along

Boundary

Increasing Angle ofIncidence

Increasing Angle ofIncidence

Total Internal Reflection

What is the angle of refraction at this point?

1. i < critical angle

2. i = critical angle

3. i > critical angle

The The critical angle critical angle is the angle at which the is the angle at which the emergent ray is refracted along the boundary emergent ray is refracted along the boundary between the two materials, (i.e. The angle of between the two materials, (i.e. The angle of refraction is 90°refraction is 90°

Diamond has a very high R.I. Diamond has a very high R.I. (2.4) which gives it a very low (2.4) which gives it a very low critical angle so light is internally critical angle so light is internally reflected many times before reflected many times before emerging. The diamond also emerging. The diamond also dispersesdisperses the light into the the light into the colours of the spectrumcolours of the spectrum

Why do diamonds sparkle so much?Why do diamonds sparkle so much?

Fibre optics are a major application of Total Fibre optics are a major application of Total Internal Reflection. Internal Reflection.

Fibre optics can be thought of as a Fibre optics can be thought of as a “wire for “wire for light”. light”. Total internal Reflection carries the light Total internal Reflection carries the light from one end of the fibre to the otherfrom one end of the fibre to the other

There are two primary examples:There are two primary examples:

1.1.Medical EndoscopesMedical Endoscopes

2.2.Fibre Optic communicationFibre Optic communication

Fibre optics consist of a Fibre optics consist of a core surrounded by core surrounded by cladding. TIR takes place cladding. TIR takes place at the core-cladding at the core-cladding boundary boundary

Core must be very optically clear (transparent) Core must be very optically clear (transparent) to to reduce absorptionreduce absorption

Cladding is a Cladding is a lowerlower refractive index than the refractive index than the corecore

Cladding prevents crossover from one fibre to Cladding prevents crossover from one fibre to another when in direct contactanother when in direct contact

Core needs to be thin to prevent Core needs to be thin to prevent Multipath Multipath DispersionDispersion

Fibre needs to be Fibre needs to be flexibleflexible

Often Often bundledbundled together together

Endoscopes are used to see inside enclosed Endoscopes are used to see inside enclosed spaces. They have many medical and other spaces. They have many medical and other uses. E.g. Internal inspection of aircraft uses. E.g. Internal inspection of aircraft structuresstructures

Air/Water Channel

Illumination Channel : contains bundle of fibres carrying incoherent light from light source

Image Channel : objective lens to form image on the end of the fibre bundle carries coherent light (fibre ends need to be in the same relative position

Tool Channel (Biopsy etc)

Today fibre optics are increasing used for high Today fibre optics are increasing used for high speed data communication. They have the speed data communication. They have the following beneficial propertiesfollowing beneficial properties

1.1.Immune to electromagnetic interference (noise!)Immune to electromagnetic interference (noise!)

2.2.No electrical current so no heating effectNo electrical current so no heating effect

3.3.Lower losses per unit length : Allows longer distances Lower losses per unit length : Allows longer distances between repeater amplifiersbetween repeater amplifiers

4.4.No corrosionNo corrosion

5.5.Higher Bandwidth, (more data to be transmitted)Higher Bandwidth, (more data to be transmitted)

Direct Stretched Data pulse out

Fibre Optic

Data pulse inReflected

If the core is wide then light travelling directly along the axis of the fibre (red) travels a shorted distance than the light which is repeatedly internally reflected (blue). This can stretch data pulses sent down the fibre and cause corruption of the data

White light is a mixture of all colours of the spectrum. Spectral dispersion can also occur if white light is used :

Violet light travels more slowly than red light. This difference in speed causes data pulses to widen which could lead to data corruption

To resolve this monochromatic light, (light of a single wavelength) is used