For a thin film (refractive index 1.4, thickness 250 nm), which visible wavelength shows constructive interference in reflected light (assume one π phase change at top surface)?
A350 nm
B467 nm
C700 nm
D850 nm
Answer & Solution
Correct answer: B. 467 nm
Condition with one π phase change: 2nt = (m + 1/2)λ. Here 2(1.4)(250) = 700 nm = (m + 1/2)λ. For m = 0: λ = 1400 nm (infrared). For m = 1: λ = 700/1.5 = 467 nm (blue, visible). For m = 2: λ = 700/2.5 = 280 nm (UV). The only visible match is 467 nm.
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