Path difference between two light waves at a point $P$ for constructive interference will be

Two light waves of intensities $I_0$ and $9I_0$ superpose at a point to produce a resultant intensity of $7I_0$. Calculate the phase difference between the light waves.

In the phenomenon of interference,energy.......

Which of the following is the path difference for destructive interference?

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):

Two coherent sources of intensities $I$ and $4I$ interfere. The maximum and minimum intensities of the resultant wave are: