In Young's Double Slit Experiment (YDSE), inserting media of refractive indices μ₁ and μ₂ in front of the slits introduces an additional path difference. The fringe shift direction depends on which slit has the higher optical path (μ × thickness). The central fringe shifts towards the slit with the higher optical path to compensate for the extra path length.

The net path difference introduced is (μ₂ - μ₁)t. If μ₂ > μ₁, the fringe pattern shifts down (towards S₂). If μ₁ > μ₂, it shifts up (towards S₁). If μ₁ = μ₂, there is no shift.

Final Answer: The correct statements are: "If μ₁ = 1.4 and μ₂ = 1.5, fringe pattern shift down" and "If μ₁ = 1.8 and μ₂ = 1.9, fringe pattern shifts up".