A double-slit experiment is set up using a helium-neon laser (l = 633 nm). Then a very thin piece of glass (n = 1.50) is placed over one of the slits. Afterward, the central point on the screen is occupied by what had been the m = 10 dark fringe. How thick is the glass? I have the solution's attached. My question is: why does the textbook not use snell's law? Is it making the simplistic argument that light traverses the glass piece perpendicularly? Would the problem be somehow changed otherwise?