alleryFor thin-film interference (oil on water), constructive interference for reflection condition:
A2nt = mλ
Bnt = mλ
C2nt = (m + 1/2) λ (with phase change at top surface)
D2nt = m × 2λ
Answer & Solution
Correct answer: C. 2nt = (m + 1/2) λ (with phase change at top surface)
Thin film of thickness t, refractive index n.
**At top surface (air-oil):** reflection has π phase change (rare-to-dense).
**At bottom (oil-water):** often no phase change.
**Constructive:** 2nt = (m + 1/2)λ.
Different conditions give different colored films.
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