We are using the GLUT solid sphere model. Normals are included with the model's geometry. Although the normals are pointing outward from the sphere, illumination of the inside is accomphished by using two-sided lighting (see Display Function).

As we know, whenever lighting is enabled, we must use materials to set the colors applied to the geometry. Since the user can view both the outside and the inside of the sphere, we set different materials for the two sides. In particular, we set a color for the GL_FRONT material and one for the GL_BACK material.