Two light sources are said to be coherent if they are obtained from

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?........$^o$

$PQ$ represents a wavefront and $AO$ and $BP$ are the corresponding two rays. Find the condition on $Q$ for constructive interference at $P$ between ray $BP$ and the reflected ray $OP$.

Two coherent light sources $A$ and $B$ are at a distance $3\lambda$ from each other (where $\lambda$ is the wavelength). Find the distances from $A$ on the $x$-axis at which constructive interference occurs.

The difference in the number of wavelengths, when yellow light propagates through air and vacuum columns of the same thickness, is $1$. The thickness of the air column is (Refractive index of air $\mu_a = 1.0003$, Wavelength of yellow light in vacuum $\lambda_0 = 6000 \text{ Å}$)

If the distance between a point source and a screen is doubled,then the intensity of light on the screen will become: