"Doctor, Doctor, I keep seeing spots before my eyes"
"Have you ever seen an optician?"
"No, just spots".
The concept of seeing an optician floating across my field of view is a scary one indeed. However, the concept of seeing spots doing the same is one I'm coming to terms with.
I had a talk to an opthamologist about this last week, as part of an eye check-up. He was very good, I have to say, and we discussed in detail some optical physics, particularly with regard to the astigmatism in my right eye (and why no pair of glasses ever seems quite right). He also reassured me that seeing floaters is nothing, in itself, to be worried about. It's basically a sign of getting old. How nice. He did though talk about signs of a detached retina to look out for (pun intented) – and did some more extensive than usual examination.
So what are those floaty things I see? To use a technical biological phrase, they are small lumps of rubbish that are floating around in the vitreous humour of the eye. They are real things – not an illusion – although I don't 'see' them in the conventional manner that I would see other objects.
The eye is there to look at things outside it. Its lens focuses light from objects onto the retina, where light sensitive cells convert the image to electrical signals that are interpreted by the brain. But given that the floaters are actually between the lens and the eye, how am I seeing them?
There are a couple of phenomena going on. First of all, a floater can cast a small shadow onto the retina. You can see this effect by using a lens to put an image of something (e.g. the scene outside) onto a piece of card, and then put something between the lens and the image. Some of the light can't get to the card, and so part of the image is shadowed. The appearence of the shadow depends on how close the object is to the card – if its right by the lens there will be very little effect – but if close to the card there'll be a tight, well-defiined shadow. My experience is that these spots are definitely most noticable in bright conditions – presumably because the shadows on the retina then appear in much greater contrast than under dull conditions.
Secondly, however, they can bend the light. Their refractive index will be different from that of the vitreous humour, and therefore when a light ray hits a floater it will bend, a little. The consequence is a defocusing of a little bit of the image, which wil be visible. If the floater stayed still, it would probably barely be noticable, but when it moves, the little bit of bluriness moves with it, and the brain picks up the movement rather effectively.
The most interesting thing to me is that it just isn't possible to look at these things. When I try, my eyes move, and consequently these bits of rubbish flit out of view. Rather like quantum phenomena, you can't observe them without changing where they are and where they are moving to.