I'd like to know why a ray of light passing through the optical center of a convex lens does not get refracted at all. According to my knowledge, a ray of light will not get refracted if the angle of incidence is zero i.e. it is along the normal. With this in mind, I see how a ray travelling along the principal axis and passing through the optical center will not get refracted at all. But why should a ray that is not travelling along the principal axis not be refracted just because it passes through the optical center. I'm almost 100% sure that it doesn't travel along the normal so why doesn't it get refracted like it should?