The space between the crystalline lens of the eye and the retina is filled with a transparent, dense material, which is called vitreous.

In a newborn baby, the vitreous is compact and firm, like jelly and it is fixedly attached to the retina. However, as someone grows older the vitreous becomes less firm, fluidizes and may be detached from the back of the eye. This phenomenon is called vitreous detachment. It is a very common and usually harmless condition.

As the vitreous tends to be detached from the retina, light flashes or floaters usually appear in someone's vision. More specifically, floaters are caused by tiny pieces of vitreous cells, which "swim" in the vitreous space and create shadows on the retina. Flashes occur when the vitreous's pulling movements act on a sensitive retina.

More serious causes of floaters and illuminations also exist. Tears, retinal detachment, inflammation, hemorrhage or a head injury may lead to floaters.

Occasionally flashes are caused by neurological problems like a headache or a migraine.

Migraine flashes occur in both eyes and usually last for 20 to 30 minutes till the headache beginning.