Two light rays, initially in phase and having wavelength = 6.00 x 10-7 rn, pass through different plastic layers of the same thickness, 7.00 x

Two light rays, initially in phase and having wavelength λ = 6.00 x 10-7 rn, pass through different plastic layers of the same thickness, 7.00 x 10-6 m. The indexes of refraction are 1.65 for one layer and 1.49 for the other.
(a) What least multiple of λ gives the phase difference between the rays when they emerge?
(b) If those two rays then reach a common point with the same amplitude, does the
interference result in complete darkness, maximum brightness, intermediate illumination but closer to complete darkness, or intermediate illumination but closer to maximum brightness? If the two rays are, instead, initially exactly out of phase, what are the answers to
(c) Part (a) and
(d) Part (b)?