In a standard YDSE apparatus a thin film μ=1.5,t=2.1μm 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, λ=4500A0 .(Symbols have usual meaning)

Shift of Central Bright due to insertion of slab,  x=Ddμ−1t ⇒x=2.1×10−6×(1.5-1)10−3 ⇒x=1.05×10−3=1.05 mm  Also, fringe width β=Dλd=1×4500×10−1010−3=45×10−5=0.45×10−3=0.45 mm   Distance between 2B and Mid point = 1.05 - 0.90 = 0.15 mmDistance between 3B and Mid point = 1.35 - 1.05 = 0.30 mmHence, Maxima nearest to Mid point is 2B. (i.e, 0.15mm)