What does a sudden onset of an arc of slashing light on the periphery of one eye indicate?

It is urgent and you need to see your ophthalmologist very soon. The inside of an eye is filled with a thick, clear jelly called the vitreous humor. As we age, the vitreous humor loses some of its water content and shrinks. At that point it is no longer big enough to fill the entire inside of the eye and it pulls away from the retina. As it pulls away, it often tugs on the retina and that causes an arc-like flash of light in the periphery of the vision. A small number of people who undergo this normal process will develop a tear in the retina or a detachment of the retina. This can be vision threatening and could result in blindness if not treated soon enough. Anyone with the symptoms of flashes of light with or without new black spots (new vitreous floaters) in their vision must be examined by the ophthalmologist right away.

This question was originally answered on Mar. 10, 2014.