In a standard YDSE apparatus a thin film $\left(\mu =1.5,t=2.1\mu m\right)$ is placed to front of upper slit. How far above or below the center point of the screen is nearest maxima located? Take D = 1m, d = 1 mm, $\lambda =4500\stackrel{0}{A}$ .(Symbols have usual meaning)

$$\begin{gathered} Shift of Central Bright due to insertion of slab, \\ x=\frac{D}{d}\left(\mu -1\right)t \\ \Rightarrow x=\frac{2.1\times {10}^{-6}\times (1.5-1)}{{10}^{-3}} \\ \Rightarrow x=1.05\times {10}^{-3}=1.05mm \\ Also, fringe width \beta =\frac{D\lambda }{d}=\frac{1\times 4500\times {10}^{-10}}{{10}^{-3}}=45\times {10}^{-5}=0.45\times {10}^{-3}=0.45mm \end{gathered}$$

Distance between 2B and Mid point = 1.05 - 0.90 = 0.15 mm

Distance between 3B and Mid point = 1.35 - 1.05 = 0.30 mm

Hence, Maxima nearest to Mid point is 2B. (i.e, 0.15mm)