I've just spent an interesting hour down at my optometrist's rooms, having my eyes looked into. And learning a whole lot of new stuff.
I've had glasses for years, since I was 10 or so. (My primary school teacher picked up the fact that I was having trouble seeing the blackboard & sent a note home with me for my parents. I was horrified at the prospect of wearing glasses - that 'four-eyes' name really stings when you're 10 - & hid the note in the hedge. Which of course led to all sorts of trouble when the teacher followed up on my parents' apparent inaction...) Over the years my myopia (short-sightedness) has stabilised & in fact these days I don't need to use specs for close work.
Anyway, I trundled off to the optometrist because yesterday I started getting these weird flashes of light in one eye: quite bright arcs of light off to the side, usually when I moved my head quickly. They were a bit like what you get when you press on your eyeball gently - that sort of pressure-generated flash of coloured light. Very puzzling & also very distracting (& a bit of a worry: an elderly neighbour of ours, years ago, had suffered from a detached retina & of course you tend to think the worst when something odd happens to your own eye).
After putting some drops in my eye to dilate the pupil (& I'm still going round with uneven pupils!) the optometrist had a good look around inside, using lenses & some very bright lights - so bright in fact that I was seeing after-images of the blood vessels over my own retina. A bit like cracked mud on the bottom of a dried-out pond, with the blood vessels being the cracks. She was able to tell me that it wasn't the retina coming away that was causing the strange arcs of light, which was a big relief. Instead, the jelly in my eye is going goopy.
The main part of your eye, back of the lens, is filled with a firm jelly-like substance called the vitreous humour (to distinguish it from the aqueous humour between lens & cornea, which as its name suggests is watery). Apparently what can happen as you age is that the vitreous humour can begin to liquefy, pulling away from the 'wall' of the eye & folding in on itself. (My optometrist was quick to reassure me that this wouldn't necessarily result in any deterioration of my vision.) However, this mass of dense jelly is attached to the retina, most firmly at the blind spot & also around the anterior edge of the retina (towards the front of the eye). And when it pulls away from the retina, the pressure changes can generate nerve impulses that your brain interprets as arc-like flashes of light (like the pressure-induced flashes I mentioned earlier). Which is kind of cool, really. There's a name for it too: posterior vitreous detachment.
Apparently the whole process can take up a few weeks, so I guess I'll be having my own intermittent private light show for a while yet :-)
(It seems there's a small risk of retinal detachment following on from vitreous detachment, especially along that front edge of the retina. My friendly optometrist told me that this can manifest as a shower of 'floaters' - she described it most graphically as looking through a shaken snow-globe. Now that's a light show I can do without, & hopefully will never experience!)