State the mathematical expression for the position yn′ of the nth dark fringe measured from the central maximum on the screen in a Young’s double-slit experiment (slit separation d, screen distance D, wavelength λ).
a) yn′=dnλD
b) yn′=d(n+21)λD
c) yn′=2d(n+21)λD
d) yn′=d(2n+1)λD