1. Key areas
Critical angle and total internal reflection.

2. What we will do today:
State what is meant by Total Internal Reflection and the Critical Angle.
State the relationship between the refractive index and the critical angle.
Carry out calculations on the above.

3. Total Internal Reflection and Critical Angle

4. Revision from last day sin θ1 / sin θ2 = λ1 / λ2 = v1 / v2
n = sin θ1 / sin θ2
We can express this as:
n2 / n1 = sin θ1 / sin θ2
NB The refractive index of air is 1.

5. Experiment Draw round a semi-circular block.
Draw in the normal line through the centre.
Shine a light ray along the normal to the centre of the block.
Continue to shine light ray at the centre and slowly move the light ray outwards (diagram 1).
Keep going until the angle of refraction is 900 (diagram 2).
Measure the angle of incidence at this point.

6. Total Internal Reflection
Diagram 1 – light is refracted.
Diagram 2 – Light is refracted at 900, the angle of incidence in this case is called the critical angle, Θc
Diagram 3 – Any angle bigger than the critical angle will show Total Internal Reflection

7. The Critical Angle
The critical angle,Θc is the angle of incidence when the angle of refraction is 900.
It is the smallest angle of incidence above which Total Internal Reflection occurs. It is often given the symbol, Θc

8. Total Internal Reflection
Takes place when all of a light ray is completely reflected and none of it is refracted.
This takes place at angles above the critical angle, Θc

9. Curved Surface
Note that there is no refraction at the curved surface because a radial ray strikes the surface at normal incidence (i.e. perpendicular - 90º).
This is why a semi-circular block is used to find the critical angle.

10. Uses of Total Internal Reflection
Total internal reflection is used to send light signals along optical fibres.
This can be used to send telecommunications such as internet, TV and phone.
This is how Virgin Media provide their services.

11. Critical Angle and Refractive Index
sin θc

12. Example n = 1 sin Θc n = 1.4 Θc = ? 1.4 = 1 sin Θc = 1 1.4
1.4 = 1
sin Θc = 1
1.4
Θc = (sin-1) 1
Θc = 45.6o
n = 1.4
Θc = ?

13. 2008 Qu: 16

14. 2009 Qu: 15

15. 2001 Qu: 27(b)

16. 2002 Qu: 27

17. 2002 Qu: 27

18. Revised Higher 2014 Qu: 15

19. Revised Higher Past Paper Questions