If two light waves reaching at a point produce destructive interference,then the condition of phase difference is:

The intensities of two coherent light waves are $I$ and $4I$. The maximum intensity of the resultant wave after interference is: (in $I$)

Given below are two statements: one is labelled as Assertion $A$ and the other is labelled as Reason $R$.
Assertion $A$: The phase difference of two light waves changes if they travel through different media having the same thickness,but different indices of refraction.
Reason $R$: The wavelengths of waves are different in different media.
In the light of the above statements,choose the most appropriate answer from the options given below.

Two identical radiators have a separation of $d = \lambda /4$ where $\lambda$ is the wavelength of the waves emitted by either source. The initial phase difference between the sources is $\pi /4$. Then the intensity on the screen at a distant point situated at an angle $\theta = 30^\circ$ from the radiators is (here $I_o$ is the intensity at that point due to one radiator alone):

In an interference experiment,the phase difference for points where the intensity is minimum is $(n=1, 2, 3, \ldots)$

Two waves having intensity $I$ and $9I$ produce interference. If the resultant intensity at a point is $7I$,what is the phase difference between the two waves?........$^o$