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Interference ( Physical Optics )

Chapter 35. Interference ( Physical Optics ). What is physical optics. It is the study of light as waves. Geometric optics treats light as particles (or rays) that travels in straight lines.

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Interference ( Physical Optics )

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  1. Chapter 35 Interference (Physical Optics )

  2. What is physical optics • It is the study of light as waves. • Geometric optics treats light as particles (or rays) that travels in straight lines. • Physical optics (wave optics) deals with the wave nature of light, such as the spreading of waves (diffraction) and the interference of waves.

  3. Adding waves Constructive interference Destructive interference Combining two waves does not always give you a bigger wave!

  4. Destructive Shift by half a wavelength Constructive

  5. Constructive interference Constructive interference occurs whenever the path difference is an integral multiple of λ: λ

  6. Destructive interference Destructive interference occurs whenever the path difference is an half-integral multiple of λ: λ

  7. The same is true in 2d (constructive)

  8. The same is true in 2d (destructive)

  9. Phase difference and path difference

  10. Interference on water

  11. Ripples

  12. It is all about path difference

  13. Young’s Double Slit (1800)

  14. What are the fringes? The bright fringes represents regions of high intensity I The dark fringes represents regions of low intensity.

  15. Why is I high here? r1 r2 Constructive Interference! the middle Light can reach this place via two paths, r1 and r2. Path difference is:

  16. What about the other fringes?

  17. Bright and dark fringes

  18. Calculating the angle

  19. What you see

  20. Example Given λ = 700nm and d = 3500nm, find the angles at which you can find the 0th, 1st and 2nd order maxima. What if you double d? (Notation: m = order ) d

  21. Radio broadcast The frequency f =1.5×106Hz, from two antennas shown below. In what directions will you find the strongest signal?

  22. Position on the screen

  23. Position on the screen (Approximation)

  24. Position on the screen (Approximation)

  25. Find the wavelength

  26. Find the wavelength

  27. Find the wavelength (Alternative)

  28. Phase difference and Path Difference

  29. Intensity

  30. Wavelength in different medium

  31. Wavelength in different materials

  32. Phase shift during reflection

  33. Watch out for the phase shift Relative phase shift ΔΦ =π No relative phase shift ΔΦ =0

  34. Example Assuming θ =0°, λ=550nm, nMgF = 1.38, what is the smallest L so that the two light rays shown will cancel out each other?

  35. Example: Thin-film interference 1 Is the fringe at the line of contact bright or dark? Given l=10cm, h=0.02mm andλ=500nm, find the fringe spacing.

  36. Example: Thin-film interference 2 What if we change the upper plate to a plastic with n =1.4, and fill the wedge with a grease of n =1.5, while the bottom is a glass with n =1.6?

  37. Other examples of interference

  38. Soap bubbles

  39. Newton’s rings

  40. X-rays diffraction

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