A relative afferent pupillary defect (RAPD) is seen on examination by shining light in one eye, then the other.
If the left eye has an RAPD it will show little constriction of the pupil when a light is shone into that eye, but when the light is shone into the right eye, the left constricts.
When swinging back to the left eye it paradoxically dilates, as it is not as responsive to stimulation (as the optic nerve is damaged).
(? See the pupil section for details ?)