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There is a gel material (called the vitreous) in the back of the eye that we are all born with. As we age (it can also happen if you are very nearsighted or suffer a blow to the head), the gel breaks apart into little pieces. Many people can see these pieces moving around as floaters. The gel is attached to the retina in many places. As it is breaking up, sometimes it can tug on the retina. This will cause you to see a flash of light. This entire process is called a posterior vitreous detachment and is harmless.
However, when the gel tugs on the retina, rarely it can tear the retina and this can then lead to a retinal detachment (which is a major problem that requires surgery to repair). The symptoms of a retinal detachment (floaters and flashes and shadows in your vision) can be the same as that of a vitreous detachment. Therefore, I tell all patients with these symptoms that they need to have their eyes checked immediately to make sure that the retina is OK.
Another cause of flashing lights is a migraine episode that can cause people to see flashing of lights for 15 to 20 minutes. This may or may not be followed by a headache.
Either way, you should have your eye checked as soon as you can.
Hope this helps.
I'm sure he would not mind checking to make sure your retina was OK.