1. Interference

2. Diffraction When waves pass through a narrow gap they spread out or………
The amount of diffraction depends on the size of the gap compared with the size of the
wavelength
Link to Diffraction animation 1 barrier and gap
link to wave interference

7. light diffraction animation
link to diffraction animation
light diffraction animation

8. Extension: Why does a single slit produce an interference pattern?
Each half of a narrow slit acts like a separate source of waves.
Light from the top half will interfere with light from the lower half

9. NODE

12. Smaller slit…wider diffraction

13. Wave Interference

14. We first look at interference of sound waves
LINK TO phet sound
Recall the definition of path difference.
The extra distance one wave has to travel to reach the observer

15. Recall the definition of constructive interference and destructive interference.
When two waves arrive in phase (pd = nλ) you get constructive interference.
When two waves arrive out of phase
{pd = (n – ½) λ} you get destructive interference.

17. Explain what causes the loud and quiet spots.

18. Why do your amplifier and speakers have black and red terminals?

19. Noise Canceling Headphones
These work by producing a sound wave that is out of phase with the external sound reaching your ear.

20. Interference of Light
When light shines through a double slit a similar phenomenon occurs:

21. Explanation

22. the waves arrive the waves arrive in phase This produces a big wave
Link to 2 source interference / path difference
This shows that:
when the path difference is
the waves arrive
in phase
This produces a
big wave
(constructive interference)
when the path difference is
the waves arrive
out of phase
This produces
no wave
(destructive interference)

23. Young’s Equation

24. L nλ θ θ x
Consider the big green triangle
Consider the small green triangle

25. Can be used for any situation
Can only be used for small angles (not diffraction gratings)

26. L nλ θ θ x
What happens to θ when λ increases?
What happens to θ when d increases?

27. Can be used in any situation
Θ means …………
n means …………
λ means …………
d means …………
Can only be used where θ is small

28. This is what the formula predicts ……………
Longer wavelength
Larger angle
A larger angle between antinodes means the antinodes are ………
Further apart

29. L θ x L θ x

31. Larger slit separation
This is what the formula predicts ……………
Larger slit separation
smaller angle

32. d = 0.20 mm
d = 0.15 mm
d = 0.10 mm

33. link to young's expt

34. Why does a longer wavelength light have an antinode at a larger angle???

35. link to ripple tank applet
We can simplify the diagram by considering one ray from each slit.
Path difference is zero
Incoming light
These rays meet to produce the antinode
central

36. λ λ Remember that the light diffracts through the hole
These rays go to the antinode
first
Incoming light θ λ λ
Path difference is

37. λ λ A longer wavelength produces the first antinode at a greater angle
These rays produce the antinode
First
Incoming light θ λ λ
Path difference is
A longer wavelength produces the first antinode at a greater angle

38. Path difference is λ θ
Path difference is λ θ

39. Why does a larger slit separation produce an antinode at a smaller angle???

40. Bigger slit separation = smaller angle
Path difference is λ θ
Incoming light
Bigger slit separation = smaller angle

41. Link to factors affecting Interference

42. Diffraction Grating
Diffraction gratings have lots of slits very close together
Typically 500 slits/mm
This means 1/500 or 0.002mm apart.

43. Diffraction Grating Equation
N is the number of slits per metre

44. 100 slits/mm d= mm
300 slits/mm d= mm
600 slits/mm d= mm

45. We have been talking about the slit separation, not slit width.
Note:
We have been talking about the slit separation, not slit width.
In Young’s Equation, “d” is the distance between the centres of the slits. (slit separation)
slit width
slit separation

46. The width of the pattern is the same.
If you move the slits closer the angle between the antinodes in the interference pattern increases.
Bright fringes
Closer slits
The width of the pattern is the same.

48. If you decrease the width of each slit, you get more diffraction
If you decrease the width of each slit, you get more diffraction. the angle between the antinodes is the same, but the pattern is wider.
Bright fringes
narrower slits

49. As the width of each slit is narrowed, the whole pattern is wider
(ie there are more fringes, same spacing)

50. This time the slit width is constant
This time the slit width is constant. BUT the distance, d, between the slits is varied

52. How could you make the “harp strings” closer together?

53. Diffraction Grating and White Light
Central antinode
white
1st antinode
Full spectrum

54. White light passes through slit and diffracts.
Diffracted beams overlap
Different colours (wavelengths) have their 1st antinode at different angles
Full spectrum is formed

55. Reflection from a CD

57. Interference of a CD

58. Interference of a CD

59. Beats
Listen to two tuning forks with slightly different frequencies……………
Describe what you hear.
use two big tuning forks, put plasticene on one OR use 2 signal generators and speakers OR use Zeitnitz oscilloscope program (free off internet)

62. Every second, the maxima will overlap; these are "beats"
Every second, the maxima will overlap; these are "beats".  The beat frequency is f2 - f1 = = 1 Hz.

63. Adding Sound Waves

64. The frequency of the sound heard is the average of the two frequencies
The frequency of the variation in loudness (beat frequency) is the difference between the two frequencies

65. Listener hears average of two frequencies, 441 Hz; this average is heard to go through a   maximum loudness two times each second.
This produces 2 Hz beats

66. What causes Beats? Two waves of similar frequency arrive together.
Sometimes they are in phase, and add to give a large amplitude.
Sometimes they are out of phase, and add to give zero amplitude.
The amplitude thus varies from zero to maximum periodically.
Amplitude determines loudness of a sound wave, so the sound goes loud/quiet/loud…… at a frequency of |f2-f1|