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PRACTICE / Thin-film interference

The substrate changes which thickness reinforces reflection

A film of index 1.30 lies between air and an index-1.80 substrate. Light of vacuum wavelength 520 nm is normally incident from air.

What is the smallest positive film thickness for a reflected phase maximum?

Organize the givens & assumptions
  • n₀ = 1.00
  • nf = 1.30
  • ns = 1.80
  • λ₀ = 520 nm

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. A200 nm200\ \mathrm{nm}
  2. B100 nm100\ \mathrm{nm}
  3. C400 nm400\ \mathrm{nm}
  4. D260 nm260\ \mathrm{nm}

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