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The path difference on reflected side of thin film between reflected rays BC and EF is given by:-

$Δ = 2μ + cos r ± \frac{λ}{2}$ {μ = R.I of materials, r = Angle of refraction}

For an extensive thin film; t → 0

$Δ = ± \frac{λ}{2}$ which is the condition for destructive interference

Hence, extensive thin film appears black in reflected light.

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