I am short sighted; have been since I was about eight. It was glasses for a few years, but then it started to get bad and taking it off for rugby matches ceased to be a feasible strategy if I wanted to be able to catch the ball. So the contact lenses came in, firstly only for match days and subsequently the whole time. Nowadays, quite a lot of my mates are completely unaware that I wake up each morning to a blurry vision of my ceiling, which I guess is a tribute to the general awesomeness of modern technology
The reasons for poor vision concern the mechanics of the eye; eyes consist of (among other things) a lens made from some squishy substance that means its shape can change, and the retina, a patch of light-sensitive cells at the back of the eye. The aim is to bend light, emanating from a source, so that it all focuses onto one point right on the retina. The extent to which this bending must occur depends how far away the source is. How much the light is bent depends on the thickness of the lens- if it is thicker, the light is bent to a greater degree, which is preferable if the object is close to you, and vice-versa for objects further away. Your body is able to control the thickness of the lens thanks to a couple of suspensory ligaments running around the top and bottom of the eye, which pull at the lens to stretch it out. If they pull harder, then the lens gets thinner and light is bent less, allowing us to focus on far away objects. The degree to which these ligaments pull is controlled by the ciliary muscle; when the ciliary muscle pulls, the ligaments slacken, and vice-versa. If the lens was kept at this thickness, then light coming from a source close to us would not be focused onto the retina, and instead of a nice, clean, crisp picture then we would instead see a blurry image. All this, it should be pointed out, is working on the scale of fractions of millimetres, and it’s all a very finely-tuned balance.
In the majority of people, this is no problem at all- their eye muscles work fine and keep the lens at the thickness it needs to be. However, amongst the short-sighted, the ciliary muscle is too big and so cannot relax to the extent that it can in a normal eye. This means that the suspensory ligaments do not have quite the range that they should, and are unable to pull really hard to get the lens out to its thinnest setting. When viewing objects up close, this is no problem at all; the light needs to be bent a lot and it all lines up nicely over the retina, producing a lovely, clear image. However, once objects get further away, try as the ligaments might, they just can’t get the lens thin enough to do its job properly. The end result is that light from faraway objects is bent too much, focusing it onto a point just in front of the retina rather than actually on it, and resulting in a blurry image. In some ways, it’s quite an amusing paradox; the need to wear glasses, so often stereotypically associated with nerdery and physical weakness, comes about as a result of a muscle being too big.
In long-sighted people, the situation is reversed; the ciliary muscle is too small, and is unable to exert the required force to make the lens sufficiently thick to see close-up objects. This causes light to be focused behind the eye, resulting in the same kind of blurriness and requiring the person concerned to wear reading glasses or similar for dealing with nearby objects.
And whilst we’re on the subject of reading glasses, let us pause and consider glasses and contact lenses in general. In many ways, glasses were humankind’s first tentative step into the field of biomechanics, and I am occasionally amazed that they have been around long enough for us to take them for granted so. Somehow, I find it endlessly amazing that, by looking through some special glass, I can suddenly see things properly; it all feels suspiciously like witchcraft, even if it takes only simple science and geometry to understand. It’s a commonly known fact that light, when passing through glass, slows down and bends. If we mess around looking at the geometry of the problem and apply that to light passing through a convex or concave shape, we arrive at an interesting conclusion- that a convex lens causes light to ‘turn inwards’, focusing initially parallel rays of light onto a point, and that a concave lens will do the reverse, causing light waves to spread out.
As we have seen, our eye has a convex lens built into it already to focus light onto the retina but we have already seen how this system can fail if all the finely-tuned controls are out of sorts. However, if we place another lens in front of our ‘broken’ lens, we can correct the flaws in it; if, for example, our original lens is too thick and bends light too much (as in short-sighted people), then by putting a concave lens in front of it we can bend the incoming light outwards, necessitating the light to be bent by a greater degree by the eye’s lens and allowing it to do its job properly. This, in effect, causes the light rays to be set at such an angle that it acts as if the object were positioned closer to the eye (my apologies if that sentence made no sense whatsoever), and a similar system using convex lenses can be utilised by long-sighted people. This is the principle upon which both glasses and contact lenses operate.
Then there’s laser eye surgery, in which the surgeon cuts open the eye, fires a laser at the cornea (the bit of the eye containing the lens and all the other refracting equipment) in order to reshape it, and then re-seals it. Now, if you will excuse me, I have to go and huddle under my duvet as a direct result of that image…