In Young’s double slit experiment, two slits \( S_{1} \) and \( S_{2} \) are \( d \) distance apart and the separation from slits screen is \( D \) (as shown in figure). Now if two transparent slobs of equal thickness \( 0.1 \, \text{mm} \) but index \( 1.51 \) and \( 1.55 \) are introduction in the path of beam \( (\lambda = 4000\ Å\ \) from \( S_{1} \) and \( S_{2} \) respectively. The central bright fringe spot will shift by ________ number of fringes.
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