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A square piece of thin film with RI = 1.4 has a wedge shaped section so that its thickness at two opposite sides

Subject: Applied Physics 2

Topic: Interference And Diffraction

Difficulty: Medium


A square piece of thin film with RI = 1.4 has a wedge shaped section so that its thickness at two opposite sides are t1&t2. If withλ=5800˚A, the number of fringes observed is 10, findt2t1.

1 Answer
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μ =1.4, λ = 5800 Å, number of fringe is 10

Condition for darkness is 2μtcos(r+θ) =nλ

For normal incidence and small wedge angle cos(r+θ)=1

Let nth fringe is obtained at thickness t1.

2μt1 = nλ ………………………………..(1)

Thickness t2 is where 10th fringe is observed

Therefore,

2μt2 = (n+10) λ………………………………..(2) …

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