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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.]

Asked in JEE Main 22nd Jan 2nd Shift 2025 · Slabs, media and modified set-ups

• (1) 94.8 nm
• (2) 275 nm
• (3) 137.5 nm
• (4) 68.7 nm

Step-by-step solution

Why the other options are wrong

• (1) Uses the glass index 1.45 in λ/(4μ).
• (2) Forgets to divide by the film index, taking t = λ/2.
• (4) Uses the antireflection formula t = λ/(4μ), which assumes phase changes at both surfaces.

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