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JEE Main 2025 Jan 22 Shift 2, Physics Q17: Interference by Thin Film

JEE Main2025Jan 22, Shift 2Physics
Q.

A transparent film of refractive index, 2.0 is coated on a glass slab of refractive index, 1.45. What is the minimum thickness of transparent film to be coated for the maximum transmission of Green light of wavelength 550 nm. [Assume that the light is incident nearly perpendicular to the glass surface.]

  1. A

    94.8 nm

  2. B

    68.7 nm

  3. C

    137.5 nm

  4. D

    275 nm

Solution

$ \text{For transmitted green light to be maximum, reflected green light should be minimum.} $

$ \Delta P=2\mu_0t=n\lambda $

$ \Rightarrow t=\dfrac{n\lambda}{2\mu_0} $

$ \therefore t_{\min} =\dfrac{\lambda}{2\mu_0} =\dfrac{550}{2\times2} =137.5\ \text{nm} $

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