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PRACTICE / Thin-film interference

Turn a film phase ledger into an actual detector reading

An air-backed film has index 1.50 and thickness 80 nm. Normally incident light has vacuum wavelength 720 nm. The two principal returned beams separately give 1 and 4 W/m² at the detector.

What combined intensity follows from the coherent two-beam model?

Organize the givens & assumptions
  • nf = 1.50
  • t = 80 nm
  • λ₀ = 720 nm
  • Returned intensities = 1, 4 W/m²

Coherent monochromatic light, lossless ordinary dielectric interfaces, normal or explicitly stated incidence, and the two principal reflected beams. Higher reflections are neglected unless stated; phase extrema do not imply exact zero reflectance.

  1. A9 W/m29\ \mathrm{W/m^2}
  2. B7 W/m27\ \mathrm{W/m^2}
  3. C3 W/m23\ \mathrm{W/m^2}
  4. D5 W/m25\ \mathrm{W/m^2}

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