The retina is the film made up of light-sensitive nerve fibres at the back of the eye.  This film is attached to the sclera, the outer casing of the eyeball.  There are a number of circumstances where the retina tears and/or becomes detached from this casing, such as due to trauma; or when the vitreous liquifies, contracts and moves forward, and pulls the retina with it.  Tears and detachments are painless; typically but not always they start in the peripheral retina, but left untreated spread to involve more central areas.  People who are more highly short-sighted are at greater risk of developing retinal tears and detachments.  Classical symptoms of a tear or detachment include photopsia (flashing lights), a sudden and dramatic increase in the number of floaters and the appearance of a veil or curtain across vision.  Retinal tears and detachments need to be treated urgently, within a day or so, in order to preserve central vision in particular.