1. Key areas
Absolute refractive index of a material is the ratio of the sine of angle of incidence in vacuum (air) to the sine of angle of refraction in the material.
Refractive index of air treated as the same as that of a vacuum.
Situations where light travels from a more dense to a less dense substance.
Refractive index can be found from the ration of speed of light in vacuum (air) to the speed in the material and the ratio of the wavelengths.
Variation of refractive index with frequency.

2. What we will do today:
Investigate the ratio of angles of incidence and angles of refraction for a light passing from one medium to another.

3. Refraction and Refractive Index

4. Experiment – Refractive Index of a Perspex Box
Method
Place the block on white paper and trace around its outline. Draw in the normal at the midpoint B.
With incident angle a = 10, measure the angle p, the refracted angle in the perspex.
Repeat for the other values of incident angle.

5. Experiment – Refractive Index of a Perspex Boxθ1 θ2
sin θ1 / sin θ2
10º
20 º
30 º
40 º
50 º
60 º

6. The ratio sin θ1 / sin θ2 is a constant when light passes obliquely from medium 1 to medium 2.
The absolute refractive index, n, of a medium is the ratio sin θ1 / sin θ2 where θ1 is in a vacuum (or air as an approximation) and θ2 is in the medium.
i.e. n = sin θ1 / sin θ2

7. Note that the refractive index of air is 1.
Therefore, we can write:
n2 = sin Θ1
n1 sin Θ2

8. The refractive index measures the effect a medium has on light
The refractive index measures the effect a medium has on light. The greater the refractive index, the greater the change in speed and direction.
The absolute refractive index is always a value greater than (or equal to) 1.

9. Calculating the refractive index, n, using a graph
The refractive index can also be calculated by plotting a graph of how sin θ1 varies with sin θ2
The refractive index is equal to the gradient of this graph.

10. Refractive Index and Frequency
Note that the different colours are refracted through different angles.
i.e. The refractive index depends on the frequency (colour) of the incident light.
The refractive index of the medium for blue light is greater than that for red.
This is why when white light passes through a triangular prism it is broken up into the colours of the rainbow.

11. Wavelength, λ, and velocity, v, change during refraction, but frequency does not change.
i.e. The frequency of a wave is unaltered by a change in medium.
When light passes from medium 1 to medium 2:
sin θ1 = λ1 = v1
sin θ2 λ2 v2

12. Example 1
Using information in the diagram, find the refractive index of the plastic:
θ1 = 90 – 30 = 60º
θ2 = 90 – 54 = 36º
n = sin θ1 / sin θ2
= sin 60 / sin 36
= 1.47
54º
Plastic
30º

13. Example 2
Calculate the speed of light in glass of refractive index 1.50.
(this is a common question, here the examiner assumes that you know the speed of light in air is 3x108 ms-1)
v1 / v2 = n
v1 / v2 = 1.50
(3 x 108) / v2 = 1.50
v2 = 2 x 108 ms-1

14. 2006 Qu: 15

15. 2005 Qu: 14

16. 2006 Qu: 16

17. 2005 Qu: 16

18. 2003 Qu: 16

19. 2003 Qu: 27 (2 of 2)

20. 2005 Qu: 28 (1 of 2)

21. 2004 Qu: 27

22. Past Paper Questions
Revised Higher

23. CfE Specimen Paper Qu: 12

24. CfE Specimen Paper Qu: 12

25. Solution

26. Questions
Class jotter
Section 6: Refraction of light

27. Revised Higher Past Paper Questions