JEE Advanced 2015 Paper 1

Two identical glass rods S1 and S2(refractive index=1.5) have one convex end of radius of curvature 10 cm.They are planed with the curved surfaces at a distance d as a shown in the figure figure.with their axes (shown by the dashed line)aligned.when a point source of light P is placed inside rod S1 on its axis at a distance of 50 cm from the curved face ,the light rays emanating from it are found to br parallel t o the axis inside S2.The distance d is