I am an ophthalmologist on this site. You have most probably had a posterior vitreous detachment. This is a normal ageing change in which the vitreous gel in the back of the eye liquifies and separates from the retina. As it separates from the retina it causes a pull which leads to flashes of light. Floaters also appear during this process. Black/brown ligh is the shrunken vitreous clump. Usually this goes uneventful but sometimes it may cause a retinal tear so it's important that you you see an ophthalmologist for a detailed eye examination. Thanks and best wishes.
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