A small transparent slab containing material of μ =1.5 is placed along AS2 (Fig.10.6). What will be the distance from O of the principal maxima and of the first minima on either side of the principal maxima obtained in the absence of the glass slab.


AC = CO = D, S1C = S2C = d << D


Whenever a transparent slab of refractive index μ and thickness t is inserted in the path of the ray the fringes on the screen shifts by (μ-1) t towards the slab.

Given AC = CO = D, S1C = S2C = d << D


L=d/4, μ=1.5


the optical path difference is given by



For principal maxima OPD=0, i.e.





The distance from O to principal maxima is,



The path difference for first minima is ± (λ/2)


i.e.




Since diffraction occurs when slit width is equals to the wavelength i.e. λ =d




Considering the positive side,



Considering the negative side,



The distance of first principal minima from O on the positive side is,



The distance of first principal minima from O on the negative side is,



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