A thin glass slab G1 is held over a large glass slab G2, creating an air gap of uniform thickness  $t=0.5\mu m$ between them. Electromagnetic wave having wavelengths ranging from  $0.4 \mu m to 1.15 \mu m$ is incident normally on the slab G1. When interference between waves reflected from boundaries of air gap (the two reflected waves are shown in fig as R1 and R2) was studied, it was found that only two wavelengths interfered constructively. One of these two wavelengths is  ${\lambda }_1=0.04\mu m$. Find the other wavelength (${\lambda }_2$) that interferes constructively. (In micrometers)