Sneezing is our response to an irritation of our nose. Sneezes are usually reflex actions, they occur involuntarily. One in every three to four people however sneeze in response to bright light.
Science does not fully understand why this happens yet but a theory suggests that the close proximity between the trigeminal nerve (the nerve that triggers sneezing) and the optic nerve (the nerve that constricts the pupil in response to bright light) may be the reason for photic sneezing.
When the person is exposed to bright light, the optic nerve sends impulses that cause the constriction of the pupils. These impulses are also extended to the trigeminal nerve which lies close to the optic nerve, sending a message to the brain to trigger a sneeze.
Another name for this occurrence is Autosomal-dominant Compelling Helio-Ophthalmic Outburst syndrome, or ACHOO.