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Electromagnetic Waves

Electromagnetic Waves. G6 Thin-film Interference. A. Monochromatic light source. B. Thin-film Interference Reflection on thin-films (see handout). White light source. Colours due to thin-film interference – oil film on water. Light source. A. B. Air. Section of film. Air.

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Electromagnetic Waves

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  1. Electromagnetic Waves G6 Thin-film Interference

  2. A Monochromatic light source B Thin-film Interference Reflection on thin-films(see handout)

  3. White light source Colours due to thin-film interference – oil film on water

  4. Light source A B Air Section of film Air Colours due to thin-film interference – thin film in air

  5. t Calculations involving thin films in air

  6. Thin air Wedges Considertwo glass slides placed with a piece of paper between them at one end: Fringes are seen from above. If we assume that the slides are coated so that no reflection occurs at the top of the top slide or bottom of the bottom slide, this is essentially like a thin film (of air) but of varying thickness. θ D Air wedge L

  7. Consider light of wavelength λ incident from above: Some light reflects at the bottom of the top slide and some at the top of the bottom slide. Q. What will the phase change be at these boundaries? So if they are to interfere constructively what must the path difference be? θ

  8. Now consider light incident vertically from above a point where the slide separation is d: For constructive interference to occur vertically above this point... 2d = (m + ½) λ i.e. Bright fringes will occur at d = λ , 3λ , 5λ etc 4 4 4 d θ

  9. Therefore there will be equal fringe separation x as shown below... From the small triangle below m=1 and m=2: From the original diagram: Thus the fringe spacing (of equal thickness x) is given by: (The thickness of the paper could thus be determined from x) m = 2 λ 2 m = 1 d = 5λ 4 m = 0 d = 3λ 4 x x θ = λ 2x θ = D L x = λL 2D

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