In a young double slit experiment, an interference pattern is obtained for\[\lambda =6000\overset{o}{\mathop{A}}\,\], coming from two coherent sources \[{{S}_{1}}\] and \[{{S}_{2}}\] at certain point P on the screen third fringe of dark is obtained. The n path difference \[{{S}_{1}}P-{{S}_{2}}P\]in microns is: