This patient is exhibiting the Marcus-Gunn phenomenon. When light strikes the retina, the pupillary light reflex is automatically triggered, leading to simultaneous constriction of both pupils. In the absence of adequate light entering the eye, for example following retinal detachment or optic neuritis, paradoxical dilatation of the pupils occurs. The retina receives far less light than it normally would, and the pupils dilate in order to absorb as much light as possible.