1. Optics: Total Internal Reflection
Learning Objectives
Book Reference : Pages
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 Reflection: Introduction
Total Internal Reflection is 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

3. T.I.R. : Key Concepts 1
2. i = critical angle
Less Dense
Away from Normal
Refract along Boundary
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. T.I.R. : Key Concepts 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 90°
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. T.I.R. : Applications
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

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

7. Fibre Optics 2
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

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

9. Endoscopes 2
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)

10. Fibre Optic Communication
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)

11. Multipath Dispersion
Fibre Optic
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. Spectral Dispersion
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