In a Young's double slit experiment, the intensity at a point is $\left( \frac { 1 } { 4 } \right) ^ { \text {th} }$ of the maximum intensity, the minimum distance of the point from the central maximum is $\_\_\_\_$ $\mu \mathrm { m }$. (Given: $\lambda = 600 \mathrm {~nm} , \mathrm {~d} = 1.0 \mathrm {~mm} , \mathrm { D } = 1.0 \mathrm {~m}$)