Two idenctical glass rods $\mathrm{S}_1$ and $\mathrm{S}_2$ (refractive index = 1.5 ) have one convex end of radius of curvature 10 cm . They are placed with the curved surfaces at a distance d as shown in the figure, with their axes (shown by the dashed line) aligned. When a point source of light $P$ is placed inside rod $S_1$ on its axis at a distance of 50 cm from the curved face, the light rays emanating from it are found to be parallel to the axis inside $\mathrm{S}_2$. The distance d is :