Physics Problem

Kevin Drum posts the following:

Over the past couple of weeks, I've noticed that my distance vision is a little fuzzy. Time for new glasses, you say, and you're probably right. But here's the odd thing. I keep all my old glasses, and last night I tried them all on just to see if an older prescription worked better than my current glasses. What I discovered was a little strange.

Right under my TV I happen to have two LED clocks. One uses red LEDs and the other uses blue LEDs. With my current glasses, the blue LEDs are sharp and the red LEDs are fuzzy. But when I put on glasses that are a few years old, it changes. The red LEDs are sharp and the blue LEDs are fuzzy. The difference is quite noticeable, not a subtle thing at all.

Anyone know what this is all about?

Naturally some of his commenters got the correct answer, but you should try to solve it without looking (zillions got wrong answers, of course). Hint: there are two parts to the answer, one involving physics, the other involving physiology.

UPDATE/Solution: OK, since only one person ventured this, and not really successfully. The eye is not an achromatic lens. Red and blue focus at different distances. Young people don't notice this since their eyes automatically adjust by slightly changing the focus of the lens of the eye. Old people, with stiff, inflexible lenses, can't do this.

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