A parallel beam of light travelling in air (refractive index \(1.0\)) is incident on a convex spherical glass surface of radius of curvature \(50 \, \text{cm}\). Refractive index of glass is \(1.5\). The rays converge to a point at a distance \( x \, \text{cm} \) from the centre of the curvature of the spherical surface. The value of \( x \) is __________ \( \text{cm} \).