Find the angular separation between the consecutive bright fringes in a Young's double slit experiment with blue-green light of wavelength $500 \mathrm{nm}.$ The separation between the slits is $2.0\times {10}^{-3} \mathrm{m}$.

A narrow slit $\mathrm{S}$ transmitting light of wavelength $\mathrm{\lambda }$ is placed at a distance $\mathrm{d}$ above a large plane mirror as shown. The light coming directly from the slit

and that coming after the reflection interfere at a screen $\mathrm{\Sigma }$ placed at a distance $\mathrm{D}$ from the slit.
(a) What will be the intensity at a point just above the mirror, i.e., just above $\mathrm{O}$?
(b) At what distance from $\mathrm{O}$ does the first maximum occur?