Thin Film — Condition for BrightHardQuestion 20234
Question
For a thin film of thickness t and refractive index n in air, constructive interference in reflected light occurs when (considering phase change at top surface):
- A2nt = mλ
- B2nt = (m+½)λ where m = 0,1,2...Correct
- C2t = mλ
- Dnt = mλ
Correct answer
2nt = (m+½)λ where m = 0,1,2...
Explanation
Light reflecting from the top (air-to-film) gets a π phase change (half-wavelength shift). Light from the bottom (film-to-air) does not. Net extra optical path = 2nt. For constructive interference in reflected light: 2nt = (m+½)λ (the phase difference from the extra path must cancel the π shift from reflection).