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Light of wavelength 540 nm illuminates a thin glass wedge of refractive index 1.5. Interference occurs between light reflected from the top and bottom surfaces of the wedge. By how much does the wedge thickness increase between two consecutive dark fringes?

1. 180 nm
2. 90 nm
3. 280 nm
4. 360 nm

Correct answer: 180 nm

Solution

In a thin wedge film viewed in reflection, consecutive dark (or bright) fringes correspond to a change in optical path of one wavelength: 2*mu*deltaₜ = lambda. Therefore the physical thickness change between adjacent dark fringes is deltaₜ = lambda/(2*mu) = 540/(2*1.5) = 180 nm.

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