Exploring Light Waves: Double Slit & Diffraction Phenomena

3. Plan for Today (AP Physics 2) • Notes on Double Slit and Light

4. Light rays Ocean Beach Fuzzy Shadow Diffraction of Light Diffraction is the ability of light waves to bend around obstacles placed in their path. Water waves easily bend around obstacles, but light waves also bend, as evidenced by the lack of a sharp shadow on the wall.

5. Water Waves A wave generator sends periodic water waves into a barrier with a small gap, as shown below. A new set of waves is observed emerging from the gap to the wall.

6. Conditions for Interference • The sources must be coherent • They must be in phase with respect to each other • The waves must have identical wave lengths

7. Interference of Water Waves An interference pattern is set up by water waves leaving two slits at the same instant.

8. Videos • MIT • General Background

9. Light source S1 S2 Young’s Experiment In Young’s experiment, light from a monochromatic source falls on two slits, setting up an interference pattern analogous to that with water waves.

10. The Superposition Principle • The resultant displacement of two simul-taneous waves (blue and green) is the algebraic sum of the two displacements. • The composite wave is shown in yellow. Constructive Interference Destructive Interference The superposition of two coherent light waves results in light and dark fringes on a screen.

11. s1 s1 s1 Constructive s2 s2 s2 Destructive Constructive Young’s Interference Pattern Bright fringe Dark fringe Bright fringe

12. p1 l l l p2 p3 p4 Path difference Bright fringes: Dp = ml, m = 0, 1, 2, . . . Dp = 0, l , 2l, 3l, … Conditions for Bright Fringes Bright fringes occur when the difference in path Dpis an integral multiple of one wave length l.

13. p1 p2 l n = odd p3 l n = 1,3,5 … p3 Dark fringes: Conditions for Dark Fringes Dark fringes occur when the difference in path Dpis an odd multiple of one-half of a wave length l/2.

14. Alternate (Book) Definition of Dark Fringes • Dp = (m+ 1/2)l • M = 0, +/- 1, +/- 2 . . .

15. x Path difference determines light and dark pattern. d sin q s1 d q s2 p1 y Dp = p1 – p2 Dp = d sin q p2 Bright fringes: d sin q = nl, n = 0, 1, 2, 3, . . . Dark fringes: d sin q = nl/2 , n = 1, 3, 5, . . . Analytical Methods for Fringes

16. From geometry, we recall that: x d sin q s1 d q s2 p1 y p2 Bright fringes: Dark fringes: Analytical Methods (Cont.) So that . . .

17. x s1 d sin q q s2 y n = 1, 3, 5 Dark fringes: Example 1:Two slits are 0.08 mm apart, and the screen is 2 m away. How far is the third dark fringe located from the central maximum if light of wavelength 600 nm is used? x = 2 m; d = 0.08 mm l = 600 nm; y = ? d sin q = 5(l/2) The third dark fringe occurs when n = 5

18. x s1 x = 2 m; d = 0.08 mm d sin q q l = 600 nm; y = ? s2 y n = 1, 3, 5 Example 1 (Cont.):Two slits are 0.08 mm apart, and the screen is 2 m away. How far is the third dark fringe located from the central maximum if l = 600 nm? y = 3.75 cm

19. d sin q d q d sin q = nl n = 1, 2, 3, … The Diffraction Grating A diffraction grating consists of thousands of parallel slits etched on glass so that brighter and sharper patterns can be observed than with Young’s experiment. Equation is similar.

20. 3l The grating equation: 2l l 1st order d = slit width (spacing) l= wavelength of light 6l 4l q= angular deviation 2l 2nd order n = order of fringe The Grating Equation

21. n = 2 300 lines/mm Example 2:Light (600 nm) strikes a grating ruled with 300 lines/mm. What is the angular deviation of the 2nd order bright fringe? To find slit separation, we take reciprocal of 300 lines/mm: Lines/mm  mm/line

22. l = 600 nm n = 2 300 lines/mm Example (Cont.) 2:A grating is ruled with 300 lines/mm. What is the angular deviation of the 2nd order bright fringe? Angular deviation of second order fringe is: q2= 21.10

23. Thin Films

24. Thin Films • Film thickness = t • Index of refraction = n • Light rays nearly normal to the surface

25. What occurs • 1. An electromagnetic wave undergoes a 180 degree phase change if it goes from a smaller n to a bigger n (n2 > n1). No phase change otherwise • 2.

26. Equation for Constructive Interference in Thin Films • 2t = (m + ½) * hn, • M = 0, 1, 2. . . • Rewritten

27. Equation for Destructive Interference in a Thin Film

28. Picture of Situation

29. Equation Derivation

30. Pictures of What This Looks Like

31. A compact disk acts as a diffraction grating. The colors and intensity of the reflected light depend on the orientation of the disc relative to the eye.

32. Relative intensity Pattern Exaggerated Interference From Single Slit When monochromatic light strikes a single slit, diffraction from the edges produces an interference pattern as illustrated. The interference results from the fact that not all paths of light travel the same distance some arrive out of phase.

33. a/2 a 1 2 a/2 3 4 5 Single Slit Interference Pattern Each point inside slit acts as a source. For rays 1 and 3 and for 2 and 4: First dark fringe: For every ray there is another ray that differs by this path and therefore interferes destructively.

34. a/2 a 1 2 a/2 3 4 5 Single Slit Interference Pattern First dark fringe: Otherdark fringes occur for integral multiples of this fraction l/a.

35. l = ? x = 1.5 m q y a = 0.35 mm Example 3:Monochromatic light shines on a single slit of width 0.45 mm. On a screen 1.5 m away, the first dark fringe is displaced 2 mm from the central maximum. What is the wavelength of the light? l = 600 nm

36. D Circular diffraction Diffraction for a Circular Opening The diffraction of light passing through a circular opening produces circular interference fringes that often blur images. For optical instruments, the problem increases with larger diameters D.

37. Clear image of each object Separate images barely seen d2 d1 Resolution of Images Consider light through a pinhole. As two objects get closer the interference fringes overlap, making it difficult to distinguish separate images.

38. Resolution limit d2 Separate images Resolution Limit Resolution Limit Images are just resolved when central maximum of one pattern coincides with first dark fringe of the other pattern.

39. D q Limiting angle Resolving Power of Instruments The resolving power of an instrument is a measure of its ability to produce well-defined separate images. For small angles, sin q  q,and the limiting angle of resolution for a circular opening is: Limiting angle of resolution:

40. p D q so q Limiting Angle of Resolution: Limiting angle qo Resolution and Distance

41. p q so q Eye Tail lights D Example 4:The tail lights (l = 632 nm) of an auto are 1.2 m apart and the pupil of the eye is around 2 mm in diameter. How far away can the tail lights be resolved as separate images? p =3.11 km

42. Young’s Experiment: x Monochromatic light falls on two slits, producing interference fringes on a screen. d sin q s1 d q s2 p1 y p2 Bright fringes: Dark fringes: Summary

43. The grating equation: d = slit width (spacing) q= angular deviation l= wavelength of light n = order of fringe Summary (Cont.)

44. Relative Intensity Pattern Exaggerated Summary (Cont.) Interference from a single slit of width a:

45. p D q so q Limiting Angle of Resolution: Limiting angle qo Summary (cont.) The resolving power of instruments.