What is the distance between the first bright fringe and first dark fringe in an interference pattern?(use conventional symbols)

Explanation

Distance between first bright fringe and first dark fringe is half of the fringe width. Fringe width `=(D)/(2d).lambda` where 2d= distance between the sources `lambda`= wavelenght of incident light and D = distance of screen from the pair of sources.
`:.` Required distance = `(D)/(2d).lambda xx (1)/(2) = (D)/(4d). lambda.`