It is observed that the Sommerfeld's formula for a progressive spherical wave is not covariant with respectro the rotation of the coordinate system.It contains complex partial waves that are not physical.The standing wave representation of the same formula is shown to be covariant and physical.It is applied to solve the boundary value problem of fiber optics.The radiation modes as well as the guided normal modes are obtained in closed forms.The intensity distribution in various modes is obtained.It is remarked that the mode of critical refraction is missing.