Relative afferent pupillary defect  is characteristically seen In damage to
For testing relative afferent pupillary defect (RAPD), a strong, steady light is used. 
The light is shined into one eye, and then quickly switched to the other. 
This is repeated back and forth, until one of four conclusions is reached (listed below).
Since light in one pupil causes both pupils to constrict, quickly switching from one eye to the other will give a "relative" indication of the functioning of each eye and optic nerve.
If both eyes are equally dysfunctional, no "relative" defect would be found.