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PRACTICE / Path difference

Count only the optical path a plate adds

Equal coherent beams each give 8 W/m². They initially arrive in phase. A 900 nm thick, index-1.50 plate replaces air in one beam at normal incidence; the vacuum wavelength is 600 nm.

What is the new combined intensity?

Organize the givens & assumptions
  • t = 900 nm
  • n = 1.50
  • λ₀ = 600 nm
  • Each separate intensity = 8 W/m²

Same-frequency coherent beams, parallel polarization, same observation medium and unchanged individual detector intensities, unless the prompt explicitly averages phase. Neglect reflection losses and displacement of the beam by the plate; the separate intensities remain unchanged.

  1. A00
  2. B32 W/m232\ \mathrm{W/m^2}
  3. C8 W/m28\ \mathrm{W/m^2}
  4. D16 W/m216\ \mathrm{W/m^2}

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