Optics: Total Internal Reflection

1. Learning Objectives Book Reference : Pages 193-195 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 stones

2. Total Internal Reflectionis an example of refraction (note the difference in naming!) • We’ve 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 Total Internal Reflection: Introduction

3. 2. i = critical angle Less Dense Away from Normal Refract along Boundary T.I.R. : Key Concepts 1 More Dense i i Increasing Angle of Incidence What is the angle of refraction at this point? 1. i < critical angle Total Internal Reflection Increasing Angle of Incidence i 3. i > critical angle

4. 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 90° T.I.R. : Key Concepts 2 Why do diamonds sparkle so much? Diamond 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 spectrum

5. 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 Endoscopes Fibre Optic communication T.I.R. : Applications

6. Fibre optics consist of a core surrounded by cladding. TIR takes place at the core-cladding boundary Fibre Optics 1

7. Core must be very optically clear (transparent) to reduce absorption Cladding is a lower refractive index than the core Cladding prevents crossover from one fibre to another when in direct contact Core needs to be thin to prevent Multipath Dispersion Fibre needs to be flexible Often bundled together Fibre Optics 2

8. Endoscopes are used to see inside enclosed spaces. They have many medical and other uses. E.g. Internal inspection of aircraft structures Endoscopes 1

9. Air/Water Channel Endoscopes 2 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)

10. Today fibre optics are increasing used for high speed data communication. They have the following beneficial properties Immune to electromagnetic interference (noise!) No electrical current so no heating effect Lower losses per unit length : Allows longer distances between repeater amplifiers No corrosion Higher Bandwidth, (more data to be transmitted) Fibre Optic Communication

11. Fibre Optic Multipath Dispersion Data pulse in Direct Stretched Data pulse out Reflected 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

12. White light is a mixture of all colours of the spectrum. Spectral dispersioncan 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 Spectral Dispersion