Light and Matter

1. Light and Matter (Quantum Physics)

3. Parts • Part One: Light • Part Two: Double Slit experiment • Part Three: Photoelectric experiment • Part Four: Wave-like properties of matter • Part Five: The quantum atom

4. Assessment • Test SAC in week 9 or 10.

5. Pracs • Slinkys • Ripple pond • Young’s double slit (diffraction) – Laser • Diffraction around a human hair • Double slit (microwaves) • Photoelectric effect

6. Part One: Light • Waves

7. Wave Properties Direction of travel A λ

8. Wave properties • Amplitude: The height of the wave (from the centre) Units: m • Wavelength: The length from one crest to the next. Units: m • Frequency: How many full wavelengths pass you every second. Units: Hz, s-1 • Time Period: The time it takes for a full wavelength to pass. Units: s

9. Wave Properties • The wave equations:

10. Questions Question One. Match each word with its value for microwaves Period. 2.45 GHz Frequency. 2 cm Wavelength. 4 ps Question Two. Michael is standing in the sea and notices that it takes 2.5 seconds between one wave crest passing him and the next. If the water waves are moving at 2ms-1, what is the wavelength of the wave?

11. The electromagnetic spectrum • Visible light is a wave • It has a speed of 3x108 ms-1. • It is a type of electromagnetic radiation • Other types of EM radiation are… • Gamma, x-ray, UV, IR, microwave, radio

12. The electromagnetic spectrum 500nm 700nm 400nm 10μm 100nm 1cm 100pm 1pm 100m

13. Questions • Question 3. Under each type of wave in the electromagnetic spectrum, write the wavelength in m (scientific notation) • Question 4. What is the frequency of red light (660nm)?

14. Superposition and Diffraction, and interference Ripple Pond simulation: Activities Write a sentence describing what happens in • The single source • The double source • The single slit • The double slit • The Doppler effect • Name three ways you can change the shape of the wave • Name two examples of superposition of a wave • Find a node (and write down/draw where) • Find an antinode (and write down/draw where)

15. Diffraction • When a plane wave (flat) goes around an object, or through a gap, it bends. This is diffraction

16. Diffraction

17. Interference • When two waves interfere, we can sometimes have places were only ever a crest from one wave cancels with a trough from another. This is called a destructive interference and the position where this happens is called a node. • Places where two troughs add, or two crests add constructively are called antinodes.

18. Part Two: The Double Slit Experiment

19. Prac: Microwave double slit What is the position of the first loud spot? How far (horizontally) to the first quiet spot? What is the path difference to the first quiet spot How far to the next loud spot? What is the path difference to the next loud spot Write down a rule (in terms of path difference) for finding bright (loud) spots Write down a rule (in terms of path difference) for finding dark (quiet) spots

20. The Double Slit Experiment Path Length Path Length

21. The Double Slit Experiment Peaks line up with peaks, troughs with troughs. This is an antinode (bright spot)

22. The Double Slit Experiment The path lengths are the same! The path difference is … ZERO!

23. The Double Slit Experiment What if the waves started with troughs instead of peaks?

24. The Double Slit Experiment They still constructively interfere!

25. The Double Slit Experiment What about looking at a new spot on the screen?

26. The Double Slit Experiment Which path has the longest length? What does that mean for the waves now?

27. The Double Slit Experiment Because the one on the bottom had to go further, the waves are OUT OF PHASE In this case, the are half a wavelength out of phase and destructively interfere! DARK SPOT!

28. The Double Slit Experiment The path difference is half a wavelength!,

29. The Double Slit Experiment • What would be the path difference for the next bright spot? • One full wavelength! • The next dark spot… • One and a half wavelengths • The next bright spot… • Two wavelengths

30. The Double Slit Experiment • When the path difference is an integer number of wavelengths, constructive interference occurs • (constructive interference) • When the path difference is a half-integer number of wavelengths, destructive interference occurs • (destructive interference)

31. The Double Slit Experiment • Important: The light that goes through each slit must be COHERENT. (In phase to start with) • It also must be MONOCHROMATIC (one colour, or one wavelength) • This is achieved by starting with a single coloured, coherent source of light (laser, maser, torch or sun through a pinhole and a coloured filter)

32. Back to microwave double slit • Play around and predict.

33. Young’s Double Slit Experiment • Young performed the original double slit experiment with light. • The only way he could explain the results was if light behaved as a wave.

34. Young’s Double Slit Experiment • Young (in the early 1800s) changed scientists’ view of what a wave was • Newton had thought it was a particle. • Young changed the thinking of the day to a wave

35. Qualitative effect of wavelength, distance between slits. Small wavelength Med. wavelength Large wavelength Small Gap Medium Gap Large Gap

36. Diffraction and Interference

37. Prac: • Using a laser, rulers and the formula from page before, find the width of a human hair.

38. Questions Question 5. In 1801, Thomas Young performed his now-famous interference experiment. In this experiment the wave nature of light was demonstrated. The figure below is a drawing similar to the original sketch recorded by Young at the time. The dark circles represent wave crests, and the troughs are midway between these.

39. Questions Which one or more of the following states is/are true? Constructive interference occurs a. When crests overlap crests b. When troughs overlap crests c. When crests overlap troughs d. When troughs overlap troughs

40. Questions Question 6. Before doing a double slit experiment Mr McGovern thinks that the central band should be a dark band. Ms Eastlake says that it should be a bright band. Who is correct. Outline your reasoning.

41. Questions B A Question 7. The experiment produces the band pattern above. Precision measurement by Dr McGovern shows that the path difference for band A (dark band) is 316nm larger than for band B (bright band beside A). What is the wavelength of the laser?

42. Exam Questions • Sample: Q 21 b • 2012 Q2b, c, d • 2011 Q1, Q2, Q3, Q4.

43. Part Three: Photoelectric effect • Revision: • We often measure the energy of very small things in eV • Just remember: The eV number should be bigger than Joules. eg (3.2x10-19J = 2eV) eV J

44. Photoelectric effect • Historical: • Newton had originally thought light was particles. • Young came along with his experiment that showed it must be a wave. • All good… • Until Hertz came along…

46. Photoelectric Effect • Hertz shined light on metal surfaces • He found that some colours of light made the metal give off sparks. • He discovered the COLOUR (wavelength) of the light, not the intensity (brightness) contributed to the sparks. • What the…?

47. Photoelectric Effect Metal Plates Spark! UV light

48. Photoelectric Effect Metal Plates No Spark! Red light

49. Photoelectric Effect • Why was this a problem? • Assuming that light is a wave, we could just make the red light bigger (brighter)!

50. Photoelectric Effect MORE POWER!!!