White light is incident normally on a glass surface of refractive index 1.52 coated with a MgF₂ film of refractive index 1.38. Find the minimum film thickness for which yellow light of wavelength 550 nm in air is absent in the reflected light.

Question

G1 Physics · Accepted Answer

For a thin film, use optical path 2μt cosr and include phase reversal at reflection. With one phase reversal, reflected maxima satisfy 2μt cosr=(m+1/2)λ and reflected minima satisfy 2μt cosr=mλ. Choose the least positive m when minimum thickness is asked. After substitution/checking the statements, the matching result is A: 99.6 nm. Hence the correct answer is A.