Relative Afferent Pupillary Disorder, also called RAPD, is a phenomenon in which the pupil of the normal eye shrinks while the pupil of the diseased eye dilates when a flashlight is shone alternately on both eyes at one-second intervals, and this phenomenon is called Relative Afferent Pupillary Disorder. This phenomenon is called relative afferent pupillary dyskinesia. There are two types of responses to light, as follows: 1) direct light reflex: when a flashlight is shone on one eye, the pupil of this eye shrinks rapidly, and the reflex afferent and efferent neural pathways of the illuminated eye are jointly involved in the reaction. 2) indirect light reflex: when a flashlight is shone on the other eye, the pupil of the eye that has not been illuminated shrinks rapidly, and the reflex efferent pathway is normal and the pupil of the ill eye is normal, and the pupil of the ill eye shrinks. The efferent pathway of the reflex is normal for this to occur. Relative afferent pupillary disorder is when alternating irradiation reveals that there is a problem with afferent and that efferent are equal, and this interphase of irradiation results in unequal pupil constriction. It is valuable in the diagnosis of monocular optic neuritis, ischemic optic neuropathy, and advanced glaucoma.