1. Wave Properties
Refraction, diffraction and superposition

2. The mug trick!

3. Today’s lesson
Refraction of light

4. The mug trick!

5. Refraction
When a wave changes speed (normally when entering another medium) it may refract (change direction)

6. Water waves
Water waves travel slower in shallow water

7. Sound waves
Sound travels faster in warmer air

8. Light waves
Light slows down as it goes from air to glass/water

9. Snell’s law
There is a relationship between the speed of the wave in the two media and the angles of incidence and refraction
Ray, NOT wavefronts i r

10. Snell’s law
speed in substance 1 sinθ1
speed in substance 2 sinθ2 =

11. Snell’s law
In the case of light only, we usually define a quantity called the index of refraction for a given medium as
n = c = sinθ1/sinθ2 cm
where c is the speed of
light in a vacuum and cm
is the speed of light in the
medium c
vacuum cm

12. Snell’s law
Thus for two different media
sinθ1/sinθ2 = c1/c2 = n2/n1

13. Refraction – a few notes
The wavelength changes, the speed changes, but the frequency stays the same

14. Refraction – a few notes
When the wave enters at 90°, no change of direction takes place.

15. A practical!

16. i r
Ask them to set up the experiment as above. Gradually increase the angle of incidence. What happens?

17. Data collection and processing
Table of raw results with quantity, unit, uncertainty and an agreement between the uncertainty and precision of measurements
Data correctly processed (including graphs and line of best fit
Uncertainties correctly propagated (calculated) and error bars on graph (max/min line of best fit)

18. Conclusion and evaluation
Compare result with actual result (referenced)
Discussion of possible systemmatic errors
Identification of weaknesses with relevant significance
Improvements based on weaknesses
No “waffly” terms!

19. Diffraction
Ripple Tank Simulation

20. Diffraction
Waves spread as they pass an obstacle or through an opening

21. Diffraction
Diffraction is most when the opening or obstacle is similar in size to the wavelength of the wave

22. Diffraction patterns HL later!

23. Diffraction patterns HLθ b
bsinθ = nλ
θ = λ/b (radians)

24. Diffraction
Diffraction is most when the opening or obstacle is similar in size to the wavelength of the wave

25.                                                                 
                                                                
                                                                
                                                                
Diffraction
That’s why we can hear people around a wall but not see them!

26. Diffraction of radio waves
                                                                
                                                                
                                                                
                                                                
Diffraction of radio waves

27. Superposition

28. Principle of superposition
When two or more waves meet, the resultant displacement is the sum of the individual displacements

29. Constructive and destructive interference
When two waves of the same frequency superimpose, we can get constructive interference or destructive interference. + = = +

30. Superposition
In general, the displacements of two (or more) waves can be added to produce a resultant wave. (Note, displacements can be negative)

33. INTERFERENCE
1. Constructive Interference - when the crests (or troughs) of two waves coincide, they combine to create an amplified wave.
All waves can be Reflected
The two waves are in phase with each other – there is zero phase difference between them. 33

35. INTERFERENCE
2. Destructive Interference - where the crests of one wave are aligned with the troughs of another, they cancel each other out.
All waves can be Reflected
The waves are out of phase (or in antiphase) with each other – they are half a cycle different from each other. 35

36. Superposition
Let’s try adding some waves!

37. Standing waves store energy, whereas travelling waves transfer energy from 1 point to another
The amplitude of standing waves varies from 0 at the nodes to a max at the antinodes, but the same amplitude for all the oscillations along the progressive wave is constant.
The oscillations are all in phase between nodes, but the phase varies continuously along a travelling wave.

38. Interference patterns
Ripple Tank Simulation

39. If we pass a wave through a pair of slits, an interference pattern is produced

40. Path difference
Whether there is constructive or destructive interference observed at a particular point depends on the path difference of the two waves

41. Constructive interference if path difference is a whole number of wavelengths

42. Constructive interference if path difference is a whole number of wavelengths
antinode

43. Destructive interference if path difference is a half number of wavelengths

44. Destructive interference if path difference is a half number of wavelengths
node