1. IGCSE Physics
Waves

2. Lesson 8 – TIR and optical fibres
Aims:
To recall the meaning of critical angle c
To recall and use the relationship between critical angle and refractive index: sin c = 1/n
To describe the role of total internal reflection in transmitting information along optical fibres and in prisms.
To understand the difference between analogue and digital signals.

3. When light passing out of the glass block its angle increases

4. No more than ninety degrees
As soon as the angle of refraction reaches 90, the light can no longer be refracted.
What happens to the light? Well the light is reflected back inside the material.

5. Total internal reflection

6. All the light is now reflected back.
TIR
All the light is now reflected back.
This effect is called Total Internal Reflection and can occur in many different materials.

7. Total internal reflection (TIR)
This angle is called the critical angle ( c).
The angle at which total internal reflection first occurs is called the critical angle.
 i <  c
Refraction
 i =  c
Critical case
 i >  c
Total internal reflection (TIR)
Different materials have different critical angles. Diamond has the lowest at 24°, which is why it reflects so much light.

8. Start at zero degrees and go up

9. Critical calculations
The critical angle for a material depends upon the refractive index. The higher the refractive index, the lower the critical angle. It can be calculated using the following formula:
Sin c = 1/n
Where:
n = Refractive index,
c = Critical angle at which TIR first occurs.

10. Critical angle – Example 1
Calculate the critical angle for a glass block of refractive index 1.45
sin (c) = 1 / n
sin (c) = 1 / 1.45
sin (c) = 0.69
c = 44º

11. Critical angle – Example 2
Calculate the refractive index of a material where TIR occurs at a critical angle of 37º
sin (c) = 1 / n
n = 1 / sin (c)
n = 1 / sin (37)
n = 1 / 0.60
n = 1.66

12. Critical angle – Example 3
Material
Refractive index
Critical angle
Glass
Water
Diamond
1.5
42°
1.33
49°
2.4
24°
The greater the refractive index, the smaller the critical angle.

13. Using TIR A right angled prism will bend light through 90.
Two of these prisms can be used to produce a periscope.

14. Back the way they came
A right angled prism will also bend light through 180.
This idea is used in reflective clothes and signs.

15. Fibre optics

16. Optical fibres Light is refracted as it enters the fibre.
Every time it tries to leave it is reflected back inside.

17. TIR in liquids and tubes
The light is always reflected back into the material and does not escape. Total internal reflection is used to send signals along fibre optic cables for the Internet and TV.

18. What are the applications of total internal reflection (TIR)?
Optical fibres, used in communication, use TIR.
You could be asked to draw on the path of the beam in an exam.
1. A beam of light enters the optical fibre.
2. It is refracted as it enters the fibre.
3. It travels down the fibre through repeated TIRs.

19. Endoscope
An endoscope uses total internal reflection to enable a doctor to look deep inside the body. It enables key hole surgery to take place.

20. A fibre optic is a very thin piece of glass
A fibre optic is a very thin piece of glass. It is so thin that once light enters at one end, it can never strike the inside of the glass at an angle less than the critical angle. The light undergoes total internal reflection as it passes along the fibre.
Fibre optic cable
Fibre optics have several advantages over normal electrical wires: (1) They can carry much more information than a wire, (2) They do not suffer from static and so give a clearer connection, (3) They have no electrical resistance, (4) They pose no danger of an electric shock if they break.

21. Optical v Electrical Optical Electrical Information Attenuation
Interference
Cost
Carry more
Carry less
Less
More No
Yes
More
Less

22. Advantages of Fibre optics
Low attenuation (signal loss) of wave means they can go further.
Small diameter of fibre for a high capacity channel.
Low cost of materials.
Cables may be non-conducting so no shocks.
High Security, it is hard to listen in on an optical fibre.

23. Disadvantages of Fibre optics
Need for additional conducting members in cable when electrical supplies are required for remote terminals.
They can be damaged by some ionising radiations.
Electrical cables are already in place.
Fibres not directly suited to multiple-access use.

24. Fibre optic pictures

25. Summary – TIR and optical fibres
When light passes out of a material it can be internally reflected. Refractive index and critical angle are connected by a formula: sin (c) = 1/n
Optical fibres are thin pieces of glass or plastic that light can travel through whilst being totally internally reflected.