Physics · Ray Optics

A monochromatic light beam travels in a medium with refractive index n = 1.6. It enters a stack of glass slabs from the bottom at an angle of 30°. The slabs are arranged such that their interfaces are parallel, and the refractive index of the slabs decreases gradually according to nm = n − mΔn, where nm is the refractive index of the m-th slab, and Δn = 0.1. The light ray eventually emerges parallel to the slab interface when passing between the (m − 1)-th and m-th slabs. Find the value of m.