Why does the human visual system produce a bright patch after staring at a bright light source and looking away? If a person looks at a bright light source, such as a light bulb or perhaps the sun for a period of time, and then looks away and closes their eyes, they typically see a bright patch in the shape of the light source for a period of time. The amount of time varies, but can last for many seconds. What explanation can be provided for this effect?
The persistence that you observe when you look at a bright light for a short period of time is called flash blindness. The phenomenon is related to your retina's response to the burst of light. Quoting from the Wikipedia article:


  Flash blindness is caused by bleaching (oversaturation) of the retinal pigment. As the pigment returns to normal, so too does sight. In daylight the eye's pupil constricts, thus reducing the amount of light entering after a flash. At night, the dark-adapted pupil is wide open so flash blindness has a greater effect and lasts longer.