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 $\theta = {30^ \circ }.$ The interfaces of the glass layers are parallel to each other. The refractive indices of different glass layers are monotonically decreasing as ${n_m} = n - m\Delta n,$ where ${n_m}$ is the refractive index of the ${m^{th}}$ slab and $\Delta n = 0.1$(see the figure). The ray is refracted out parallel to the interface between the $${\left( {m - 1} \right)^{th}}$$ and ${m^{th}}$ slabs from the right side of the stack. What is the value of $m$?