# Optics: Total Internal Reflection

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Learning Objectives. Book Reference : Pages 193-195. Optics: Total Internal Reflection. To understand the concept of T otal I nternal R eflection (T.I.R.) To be able to apply TIR to applications such as fibre optics and gem stones. - PowerPoint PPT Presentation

### Text of Optics: Total Internal Reflection

• To understand the concept of Total Internal Reflection (T.I.R.)To be able to apply TIR to applications such as fibre optics and gem stonesBook Reference : Pages 193-195

Lesson 2

• Total Internal Reflection is an example of refraction (note the difference in naming!)

Weve seen that when light travels from a more to a less optically dense material the light is refracted away from the normal (e.g. glass into air)At a certain incidence angle the light is refracted along the boundary between the two materials

Lesson 2

• iiiMore DenseAway from NormalLess DenseRefract along BoundaryIncreasing Angle ofIncidenceIncreasing Angle ofIncidenceTotal Internal ReflectionWhat is the angle of refraction at this point?1. i < critical angle2. i = critical angle3. i > critical angle

Lesson 2

• The critical angle is the angle at which the emergent ray is refracted along the boundary between the two materials, (i.e. The angle of refraction is 90Diamond has a very high R.I. (2.4) which gives it a very low critical angle so light is internally reflected many times before emerging. The diamond also disperses the light into the colours of the spectrumWhy do diamonds sparkle so much?

Lesson 2

• Fibre optics are a major application of Total Internal Reflection. Fibre optics can be thought of as a wire for light. Total internal Reflection carries the light from one end of the fibre to the other

There are two primary examples:Medical EndoscopesFibre Optic communication

Lesson 2

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

Lesson 2

• Core must be very optically clear (transparent) to reduce absorptionCladding is a lower refractive index than the coreCladding prevents crossover from one fibre to another when in direct contactCore needs to be thin to prevent Multipath DispersionFibre needs to be flexibleOften bundled together

Lesson 2

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

Lesson 2

• Air/Water ChannelIllumination Channel : contains bundle of fibres carrying incoherent light from light sourceImage 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 positionTool Channel (Biopsy etc)

Lesson 2

• Today fibre optics are increasing used for high speed data communication. They have the following beneficial propertiesImmune to electromagnetic interference (noise!)No electrical current so no heating effectLower losses per unit length : Allows longer distances between repeater amplifiersNo corrosionHigher Bandwidth, (more data to be transmitted)

Lesson 2

• DirectStretched Data pulse outFibre OpticData pulse inReflectedIf 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

Lesson 2

• 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

Lesson 2

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