Light from a monochromatic point source S is incident on the two slits of an opaque wall placed at a distance L as shown in the diagram, Intensity of interference pattern is observed at the point P on the screen placed at a distance L behind the slits. When the upper slit is shifted upward then the distance between the slits is changed from  $200 \mu m$  to  $400 \mu m$,  the intensity at P changes from  $I_{\max }$ to  $\frac{3}{4}{I}_{\max }$. If L=1 m, then find the wave length of light used  $\left(inA\right)$