Flashes of light
Quick answer
Seeing flashes of light — like lightning streaks or sparks — without any actual light source usually happens when the vitreous gel, as it gradually separates from the retina, pulls on it.
Seeing flashes of light — like lightning streaks or sparks — without any actual light source usually happens when the vitreous gel, as it gradually separates from the retina, pulls on it. That mechanical traction stimulates the retina, which interprets the stimulus as a flash of light.
Not all flashes are cause for urgency: some people experience them occasionally and benignly, related to the natural aging process of the vitreous. Still, with any newly-occurring flash, the safest course is to have your retina examined with pupil dilation to rule out a cause that does need treatment.