Young's Double Slit Experiment $S_{1}$ $S_{2}$ d Screen D P y O

Source

$\Delta$ = dy/D β = λD/d

Key labels: $S_1$, $S_2$ = slits (separation $d$); $D$ = screen distance; $P$ = observation point; $y$ = height from center O; fringe pattern shown on screen; $\beta = \lambda D/d$ = fringe width.