The eye is filled with a jelly-like substance know as the vitreous. The vitreous is loosely attached to the retina at the back of the eye. The retina is quite fragile and can tear. This may happen spontaneously or following a blow to the eye or head. As the retina tears, flashing lights are often seen – usually out of the corner of the eye. If this is not treated, fluid can pass through the tear and cause a larger area to become detached. This can lead to a permanent loss of vision if not treated straightaway. A tear or a detachment often causes a sudden increase in the number of floaters.