Eyeglasses and Dispersion

I took a trip to my local optometrist this afternoon as I need new glasses. Having joined the over-40 crowd a few years ago, I’ve taken to wearing bifocals. I gave up on the progressive lens version as I simply couldn’t get used to them. That’s apparently a common problem, but annoying to find out after the fact since not only could I not get used to them but they cost more, too.

However, the big deal for me is dispersion. Eyeglasses are made from a few different materials and they don’t all bend light the same way. In particular, as any astronomer knows, it’s not so much how they bend light that matters as it is how differently the bend different wavelenghts of light. When I look at the moon off axis, one side turns red, the other blue.

I thought I was going into buy new glasses and was prepared. I had learned that real glass has lower dispersion which would reduce the effect. So I asked for glass lenses. Oops, (almost) nobody uses glass any more. And according to my optometrist, cars are to blame. Or rather, car air-bags. Apparently the risk of eye injury from an air-bag impact is too high. So my choices are polycarbonate, high index plastic or regular plastic, also known as CR39. So now I’m back to the research phase before I order.