prisms in the eye along with distorted vision- what might it be?

Please tell me what this could be: Quite frequently, out of the blue, I get distorted vision which is followed by prism-like shapes- something like holding crystals in the sunlight which I see on the visual area on the outer side. There is no pain, but it is very scary while it lasts - which is about 15-20 minutes. I never know when it might happen and where I'll be when it does. Please tell me what this might be and what could I do to avoid it.