In a double slit arrangement, fringes are produced using light of wavelength $4800 \stackrel{\mathrm{o}}{\mathrm{A}}$. One slit is covered by a thin plate of glass of refractive index $1.4$ and the other with angular glass plate of the same thickness, but of refractive index $1.7$. By doing so, the central bright fringe shifts to original fifth bright fringe from the centre. Thickness of glass plate is

Two ideal slits $S_1$ and $S_2$ are at a distance $d$ apart and illuminated by light of wavelength $\lambda$, passing through an ideal source slit $S$, placed on the line through $S_2$, as shown. The distance between the planes of slits and the source slit is $D$. A screen is held at a distance $D$ from the plane of the slits. The minimum value of $d$ for which there is darkness at $O$ is

A beam with wavelength $\lambda$ falls on a stack of partially reflecting planes with separation $d$. The angle $\text{θ}$ that the beam should make with the planes so that the beams reflected from successive planes may interfere constructively is (where, $n=1, 2, ...$)