$$\mathrm{A}$$ and $$\mathrm{B}$$ are two interfering sources where $$\mathrm{A}$$ is ahead in phase by $$54^{\circ}$$ relative to B. The observation is taken from point $$\mathrm{P}$$ such that PB $$-$$ PA = 2.5 $$\lambda$$. Then the phase difference between the waves from A and B reaching point P is (in rad)

The ratio of intensities of two points on a screen in Young's double slit experiment when waves from the two slits have a path difference of $$\frac{\lambda}{4}$$ and $$\frac{\lambda}{6}$$ is

$$\left(\cos 90^{\circ}=0, \cos 60^{\circ}=0.5\right)$$

In Young's double slit experiment when a glass plate of refractive index 1.44 is introduced in the path of one of the interfering beams, the fringes are displaced by a distance '$$y$$'. If this plate is replaced by another plate of same thickness but of refractive index 1.66, the fringes will be displaced by a distance

One of the slits in Young's double slit experiment is covered with a transparent sheet of thickness $$2.9 \times 10^{-3} \mathrm{~cm}$$. The central fringe shifts to a position originally occupied by the $$25^{\text {th }}$$ bright fringe. If $$\lambda=5800$$ $$\mathop A\limits^o $$, the refractive index of the sheet is