A monochromatic light is travelling in a medium of refractive index n=1.6. It enters a stack of glass layers from the bottom side at an angle  θ=300. The interfaces of the glass layers are parallel to each other. The refractive indices of different glass layers are monotonically decreasing as  nm=n−mΔn, where nm  is the refractive index of mth the  slab and Δn=0.1 (see the figure). The ray is refracted out parallel to the interface between the  m−1th and mth  slabs from the right side of the stack. What is the value of m?