Light waves are emitted by a common source. For what time interval will the phase remain constant?

Two coherent point sources $S_1$ and $S_2$ are separated by a small distance '$d$' as shown. The fringes obtained on the screen will be

The correct formula for fringe visibility is

The resultant amplitude of coherent sources depends on ......

In an interference experiment, the path difference between two interfering waves at a point $A$ on the screen is $\lambda/3$, where $\lambda$ is the wavelength of these waves, and at another point $B$ the path difference is $\lambda/6$. The ratio of intensities at points $A$ and $B$ is . . . . . . .

For constructive interference to take place between two monochromatic light waves of wavelength $\lambda$, the path difference should be