1. AP Physics B Summer Course 2012 2012 年 AP 物理 B 暑假班 M Sittig Ch 23: Waves

2. Waves  Waves are all around us.  They are a form of simple harmonic motion.

3. Transverse and Longitudinal

8. Wave Terminology

9. Wave Speed

10. Velocity of a wave Wave speed (m/s) Wavelength (m) Frequency (1/s)

11. Practice Problem  A harmonic wave is traveling along a rope. It is observed that the oscillator that generates the wave completes 40.0 vibrations in 30.0 s. Also, a given maximum travels 425 cm along a rope in 10.0 s. What is the wavelength?

12. Practice Problem  The displacement-time graph and displacement- distance graph of a particular wave are shown below. What is the speed of the wave?

13. Interference  When two waves overlap, interference occurs.  Superposition principle: When two or more waves occupy the same space at the same time, the displacements (+ and -) add at every point on the waves. The resultant displacement at any point is the sum of the separate displacements of the waves.

14. Constructive Interference  It occurs whenever two waves combine to make one wave with a larger amplitude.

15. Constructive Interference

16. Destructive Interference  When the amplitudes of two waves combine producing a smaller amplitude. (The two waves need not cancel each other totally.)

17. Destructive Interference

18. Interference  http://physics.ham.muohio.edu/waveapplet s/pulseadd.htm http://physics.ham.muohio.edu/waveapplet s/pulseadd.htm

19. Interference AB DESTRUCTIVE CONSTRUCTIVE

20. Interference AB DESTRUCTIVE CONSTRUCTIVE

21. Standing Waves  A standing wave is a wave that appears to be stationary, even though it is really two separate waves interfering as they pass through each other.  It is due to superposition of incident and reflected waves of same frequency, amplitude and wavelength traveling in opposite directions.

22. Standing Waves

23.  http://www.phys.unsw.edu.au/jw/module1 _nodes.swf http://www.phys.unsw.edu.au/jw/module1 _nodes.swf

24. Standing Waves

26.  There are points where the displacement is always zero. These points are called nodes of the standing wave.  Points which have the greatest amplitude are called the antinodes.

27. Standing Waves

30.  Number of loops in standing wave depends on the vibration frequency. f 2f 3f 4f

31. Standing Waves in Pipes  Number of loops in standing wave depends on the vibration frequency.  This FIRST SOUND is called the FUNDAMENTAL FREQUENCY or the FIRST HARMONIC.

32. Standing Waves in Pipes  Harmonics are MULTIPLES of the fundamental frequency  In a closed pipe, you have a NODE at the 2nd harmonic position, therefore NO SOUND is produced

33. Standing Waves in Pipes  You will get your FIRST sound when the length of the pipe equals one-half of a wavelength.

34. Example Problem  The speed of sound waves in air is found to be 340 m/s. Determine the fundamental frequency (1st harmonic) of an open-end air column which has a length of 67.5 cm.

35. Practice Problem  The windpipe of a typical whooping crane is about 1.525-m long. What is the lowest resonant frequency of this pipe assuming it is a pipe closed at one end? Assume a speed of sound of 340 m/s.

36. Doppler Effect  Imagine a motorcycle or racecar, how the frequency changes (not volume).

37. Electromagnetic Waves  See EM Waves PPT.EM Waves PPT

38. Double Slit  See PPT: AP_Physics_B_-_Diffraction.pptAP_Physics_B_-_Diffraction.ppt

39. Double Slit

40. Single Slit

41. Single Vs Double

42. Double in Real Life

43. Diffraction Gratings  http://hyperphysics.phy- astr.gsu.edu/hbase/phyopt/mulslidi.html http://hyperphysics.phy- astr.gsu.edu/hbase/phyopt/mulslidi.html

44. Review  See PPT: PPA6_ConcepTests_Ch_24.pptPPA6_ConcepTests_Ch_24.ppt

45. Example Problem  A viewing screen is separated from a double slit source by 1.2 m. The distance between the two slits is 0.030 mm. The second -order bright fringe (m=2) is 4.5 cm from the central maximum. Determine the wavelength of light.

46. Index of Refraction  n, a number that describes how much light slows down (and bends) when it passes through a material.

47. Thin Films

48. Thin films  1. Count the phase changes.  2. The extra distance traveled in the material is a round trip (2t).  3. The wavelength in the material is λ n =λ/n.  4. Apply 2t = mλ n. If phase changes is even (0, 2…) then integers for m means constructive interference, half-integers means destructive. If phase changes is odd (1, 3…) then it is the reverse.

49. Example/Practice Problem  A thin film of material is to be tested on the glass sheet for use in making reflective coatings. The film has an index of refraction n f = 1.38. White light is incident normal to the surface of the film as shown below. It is observed that at a point where the light is incident on the film, light reflected from the surface appears green ( = 525 nm).  a. What is the frequency of the green light in air?  b. What is the frequency of the green light in the film?  c. What is the wavelength of the green light in the film?  d. Calculate the minimum thickness of film that would produce this green reflection.