In a YDSE, the light of wavelength $$\lambda=5000\mathop A\limits^o $$ is used, which emerges in phase from two slits at $$a$$ distance $$d=3 \times 10^{-7} \mathrm{~m}$$ apart. A transparent sheet of thickness $$t=1.5 \times 10^{-7} \mathrm{~m}$$ and refractive index $$\mu=1.25$$ is placed over one of the slits. What is the new angular position of the central maxima of the interference pattern, from the centre of the screen? Find the value of $$y$$.

In a young's double slit arrangement fringes are produced using light of wavelength 4000 $$\mathop A\limits^o $$. One slit is covered by a thin plate of glass of refractive index 1.4 and the other with another glass plate of same thickness but of refractive index 1.7. By doing so the central bright shifts to original sixth fringe from centre. Thickness of glass plate is ................. .