I am looking at my laptop screen and keyboard through glasses with high index lenses. I am very short sighted so without lenses made from high-refractive-index plastic, my glasses would look a bit like bottle tops. There is a list of lens materials here. The index is a measure of how much the light is bent by the material of the lens, the higher is the index of the lens the more light bends when it enters the plastic of the lens and so the thinner the lens can be made.

I am also of course using my eyes to look at the laptop, and my eyes also contain lenses of course. Interestingly, these are also made of a high index material – for just the same reason. Both my eyes and glasses need to bend light and the basic rules of optics mean that you need transparent materials with high indices to do this.

I don’t actually know how manufacturers of glasses make high-index lenses, but I am reading a paper by Zhao, Magone and Schuck on how we and other animals have evolved high-index lenses in our eyes. This is actually quite a feat as evolution has pretty poor materials to work with. The lenses in glasses (as opposed to the ones in our eyes) can be made from plastics with an index of around 1.7 – an index of 1 does not bend light at all and 1.7 is quite far from 1 and so bends light a lot.

But our tissues are mostly water and water has a comparatively rubbish index of 1.33 – it doesn’t bend light very much. Now our bodies also contain lots of proteins and adding proteins to water increases the index. So it appears that the cells in our eyes have evolved to have a relatively high index via two ways: the first is that they are absolutely packed with protein, to maximise the increase in index due to the protein. The second is that proteins in the lenses themselves have evolved to have unusually large amounts of sulphur – which also tends to increase the index.

The paper is about the fact that cramming lots of protein into the cells in the lens of eye is potentially problematic: it tends to increase the (osmotic) pressure. This is bad, a high index lens is no good if the cells it is made of are at such high pressure that they tend to explode.

To avoid this it looks like (we are not 100% sure) the proteins have evolved to be a bit sticky, as sticking together would reduce the pressure. A consequence of this may be our tendency to form cataracts – these form when proteins stick together to form domains inside the cell that are hundreds of nanometres across. These domains have a different index from their surroundings and this variation in index causes light to bend constantly as it goes through the lens. All this bending of the light causes the light to pinball around and so makes the lens go milky.

The proteins in octopus eyes are of different evolutionary origin than the ones in our eyes, so I repeat the question in an earlier post: Do octopuses get cataracts? I genuinely don’t know the answer, but I’d love to know.