Let's see the path

Two mirrors $M_1$ and $M_2$ make an angle $\phi$ with line $AB$ . A point source $S$ is kept at a distance r from the point of intersection of mirrors. A small hemispherical annular mask is kept close to $S$ so that no ray emanating from $S$ directly reaches the screen kept at a distance $b$ from the source. Interference pattern of rays reflected from $M_1$ and $M_2$ is observed on the screen. The fringe width of this interference pattern is (in micro-meter )

$\lambda=5000 \mathring { A }$ , $b=4~m$ , $r=1~m$ , $\phi=30^{\circ}$