The first condition that needs to be considered is a retinal tear. Although this is an uncommon cause of flashing lights, it is the most worrisome.
The retina is the delicate layer of tissue that lines the back of the inside of the eye. A retinal tear can lead to a retinal detachment, which means the retina actually separates from the tissue layer below it. Retinal detachment can lead to significant vision loss.
Retinal tears almost never occur in both eyes at the same time. Cover one eye and then the other. If you see flashing, flickering, or streaking lights in only one eye, then you should arrange for an urgent eye exam. In addition to retinal tears and detachment, flashing lights in one eye can be caused by other diseases affecting the retina or the jelly-like fluid called the vitreous in front of the retina.
A more common cause of flashing lights is migraine. Before the headache of a migraine attack settles in, many people experience visual disturbances. It may be flashing lights, sparkling or shimmering lights, or zigzag bright lines. These will be seen in both eyes because the images originate in the brain, not the eye.
After a blow to the head, a person may see flashing lights. If the lights occur in just one eye, then the blow may have caused a retinal tear. More likely, both eyes will see the flashing lights (commonly referred to as "seeing stars"), especially if the blow is to the back of the head. Visual images are processed in the back part of the brain, called the occipital lobe. In theory, the trauma can stimulate the occipital lobe to create the flashing lights.