I suddenly saw bright light and shapes similar to broken glass in my left eye. it reduced in ~1-2 min. this happened once bfore. what could be reason?

Seeing reflection of: blood vessels in the retina. Those lines are a reverse image (shadows) of what is in your eyeball, and you are seeing it at some jagged edges, when in reality, it's a vascular tree of your blood vessels in your eye, and quite normal. A bright light can be ocular migraine or can be phosphenes when you touch your eye and see a temporary light internally. This is benign in any case.