$($II$)$ Coherent light from a laser diode is emitted through a rectangular area $3\mathrm{.}0$ mm $*1\mathrm{.}5$ mm $($horizontal$-$by$-$vertical$).$ If the laser light has a wavelength of $680$ nm$,$ determine the angle between the first diffraction minima on either side, $($a$)$ above and below the central maximum, $($b$)$ to the left and right of the central maximum.