The sign convention followed in the spherical mirror is also applicable to refraction at spherical surface. In addition to this two more sign conventions to be introduced which are:
(i) The power of a converging lens is positive and that of a diverging lens is negative.
(ii) The refractive index of a medium is always said to be positive. If two refractions are involved, the difference in their refractive index is also taken as positive.
