1. Higher Physics – Unit 3
3.2 Refraction of Light

2. Refraction
When light passes from air into a material the ratio is constant. θa θm
θa = angle of incidence in air (larger)
θm = angle of incidence in material

3. θ sin n = The absolute refractive index, n, of a medium is given by:
where θ1 is the angle in a vacuum (air is used as an approximation) and θ2 is the angle in the medium. 2 1 θ
sin n =
Material
Refractive Index n
glass
1.5
perspex
1.47
water
1.33
diamond
2.4

4. A ray of light shines into a block of perspex. Calculate angle x
Example
A ray of light shines into a block of perspex. Calculate angle x
200 x

5. Worksheet – Radiation & Matter Tutorial
Q18, 19

6. Outcome 3 Refractive Index of a Perspex Block
Place the block on white paper and trace around its outline.
Draw in the normal at the midpoint B. C
normal B θp θa
With incident angle θa = 100, measure the angle θp, the refracted angle in the perspex. A
Repeat for other values of incident angle.
Use an appropriate format to determine the refractive index of the perspex block.

7. Refractive Index The refractive index can also be found using:
speed of light in air (3x108 ms-1)
speed of light in material (ms-1)
wavelength of light in air (m, nm)
wavelength of light in material (m, nm)

8. Frequency
Light of wavelength 600nm in air is shone through glass of refractive index 1.5.
Calculate: a) speed of light in the glass
b) wavelength of light in the glass
c) frequency of light in the air
d) frequency of light in the glass
2 x 108 ms-1
400nm
5 x 1014 Hz
5 x 1014 Hz

9. fair = fmaterial Conclusion
The frequency of a wave is determined by its source and does not change in different media.
fair = fmaterial

10. Snell’s Law
Refractive index depends on the frequency of the incident light.
Refraction occurs because a wave travels at different speeds in different media.
The refractive index is equal to the ratio of the speeds, giving:
but as frequency is constant this cancels to:

11. Worksheet – Radiation & Matter Tutorial
Q20, 22, 23, 24

12. Critical Angle small incident angle maximum incident angle
total internal reflection
large incident angle
normal

13. Critical Angle of a Perspex Block
normal θa θp B C
Make measurements of various incident angles θp and the corresponding refracted angle θa to determine the critical angle θc for the perspex block.

14. Critical Angle Formula
normal θc
900
When the angle in the medium is equal to the critical angle, the angle in air is 900
So applying Snell’s Law:
But because sin 90 = 1 or

15. Example
The refractive index of glass is 1.5. Calculate the critical angle.

16. Worksheet – Radiation & Matter Tutorial
Q25, 26, 27, 28