Two sources are said to be coherent if they produce waves

The intensity of light depends upon:

Two monochromatic coherent light waves of amplitudes $A$ and $2A$,interfering at a point,have a phase difference of $60^{\circ}$. The intensity at that point will be proportional to (in $A^2$)

For obtaining maximum contrast between the bright and dark fringes in an interference pattern, the intensities of the two light coherent sources should be

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):

Resultant intensity at the centre of the screen due to two coherent sources is $I_0$. If the sources are incoherent,then the intensity at the same point will be