Glass of refractive index 1.5 is coated with a thin layer of thickness t and refractive index 1.8 . Light of wavelength lambda travelling in air is incident normally on the layer. It is partly reflected at the upper and the lower surfaces of the layer and the two reflected rays interface . If `lambda = 648 nm`, obtain the least value of t `("in" 10^(-8)m ) `which the rays interface constructively.