A point object is placed at the centre of a glass sphere of diameter 12 cm and refractive index 1.5. What is the distance of the virtual image from the surface of the sphere?

Two rays 1 and 2 from the object placed at O fall normally on the spherical surface and go through undeviated (see Fig). The divergent rays 1' and 2' appear to come from O. Hence the virtual image is formed at the centre O, which is at a distance of 6 cm from the surface of the sphere.