Solving a problem in physics section 2 point 21 optics

21. The plane-parallel glass plate thickness d = 1,2mkm and the refractive index n = 1,5 placed between two media with refractive indices n1 and n2. Light with a wavelength λ = 0,6mkm normally falls on the plate. The optical path difference Δ waves 1 and 2 are reflected by the upper and lower surfaces of the plate, and indicate an increase or decrease the intensity of light interference occurs when the following conditions: 1) n1 <n <n2; 2) n1> n> n2; 3) n1nn2; 4) n1> n <n2.