In Young’s double slit experiment light emerging from two slits have intensity I and 4I. At point A on the screen phase difference between them is $\mathrm{\pi }/2$ and intensity is ${\mathrm{I}}_{\mathrm{A}}$ . At a point B on the screen phase difference between two waves is $\mathrm{\pi }$ and intensity is ${\mathrm{I}}_{\mathrm{B}}$ then ${\mathrm{I}}_{\mathrm{A}}-{\mathrm{I}}_{\mathrm{B}}$ is