For 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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