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What is the minimum thickness of a soap film needed for constructive interference in reflected light, if the light incident on the film is 750 nm? Assume that the refractive index for the film is µ = 1.33.

(a) 282 nm 

(b) 70.5 nm 

(c) 141 nm 

(d) 387 nm

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(c) 141 nm

Explanation: Here, 2µt = \(\frac{λ}{2}\)

∴ tmin = \(\frac{λ}{4\mu}=\frac{750}{4\times1.33}\) = 141 nm

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