A Young's double slit experiment is conducted in water $\left({\text{μ}}_1\right)$ as shown in the figure, and a glass plate of thickness $t$ and refractive index${\text{μ}}_2$ is placed in the path of $S_2$. The magnitude of the phase difference at $O$ is (Assume that $\text{λ}$ is the wavelength of light in air,$O$ is symmetrical w.r.t.($S_1$ and $S_2$.)

In the figure shown in a Young's double slit experiment, a parallel beam of light is incident on the slits from a medium of refractive index $n_1$. The wavelength of light in this medium is ${\text{λ}}_1$. A transparent slab of thickness $t$ and refractive index $n_3$ is put in front of one slit. The medium between the screen and the plane of the slits is $n_2$. The phase difference between the light waves reaching point $O$ (symmetrical relative to the slits) is,

Two coherent light sources, $P$ and $Q$, each of wavelength $\text{λ}$, are separated by a distance $3\text{λ}$ as shown. The maximum number of minima formed on line $AB$ which runs from $-\infty$ to $+\infty$ is,