slit of a double-slit arrangement. The central point on the G Example: A thin flake of mica(n=1.58)is used to coveror screen is occupied by what used to be the seventh bright fringe. If)=5500A, what is the thickness of the mica? Solve: nb X△=(r2-b+nb)-r2 7 (n-1)b=7X b=7M(n-1) D 7×5500×10 6.6 μmExample：A thin flake of mica(n=1.58)is used to cover one slit of a double-slit arrangement. The central point on the screen is occupied by what used to be the seventh bright fringe. Ifλ= 5500Å, what is the thickness of the mica? S1 S2 d D X O P x 1 r 2 r nb O Solve: Δ=(r2 -b+nb)-r2 = 7λ (n-1)b = 7λ b = 7λ/(n-1) = 7550010-4 = 6.6m