A glass plate of refractive index \(1.5\) is coated with a thin layer of thickness \(t\) and refractive index \(1.8\). A beam of light of wavelength \(648 nm\) travelling in air is incident normally on the layer. It is partially reflected off the upper and the lower surface of the layer, and the two reflected rays interfere.
Find the minimum value of \(t\) in \(nm\) for which the two reflected rays interfere constructively.
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