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Solution


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.]          [2025]
1 94.8 nm  
2 68.7 nm  
3 137.5 nm  
4 275 nm  

Ans.

(3)

For maximum transmission, reflection is zero.

∴  For green light there should be minima for interference of two reflected lights.

For transmitted green light to be maxima, reflected green should be minima.

△x=2μ0t=nλ

⇒ t=nλ2μ0

∴  tmin=λ2μ0=5502×2=137.5 nm