In Young's double slit experiment, bright fringes are of,

In Young's double slit experiment, the two slits act as coherent sources of equal amplitude $A$ and wavelength $\lambda$. In another experiment with the same set up the two slits are of equal amplitude $A$ and wavelength $\lambda$  but are incoherent. The ratio of the intensity of light at the mid-point of the screen in the first case to that in the second case is 

In Young's double-slit experiment, if the widths of the slits are in the ratio $4:9$, the ratio of the intensity at maxima to the intensity at minima will be

If a transparent medium of refractive index $\mu =1.5$ and thickness $t=2.5\times {10}^{-5} \mathrm{m}$ is inserted in front of one of the slits of Young's double-slit experiment, how much will be the shift in the interference pattern? The distance between the slits is $0.5 \mathrm{mm}$ and that between slits and screen is $100 \mathrm{cm}$.

In a Young's double slit experiment, a slab of thickness $1.2 \mathrm{\mu m}$ and refractive index $1.5$ is placed in front of one slit and another slab of thickness $t$ and refractive index $2.5$ is placed in front of the second slit. If the position of the central fringe remains unaltered, then the thickness $t$ is,