I rarely wear glasses. They’re required for flying, and they’re helpful at night – especially if I’m tired; but generally they’re not needed. I have noticed this, though: When I do decide to wear my glasses with Transition lenses when I drive, they don’t get dark. I’ve heard (back in the '80s) that lenses like this need to be ‘primed’ initially by exposing them to sunlight. My first pair (this is my second) darkened nicely. The new ones don’t. I’ve tried putting the glasses in a sunny place on the dashboard, and they’ve never darkened much.
Just now I put the glasses on the patio since it’s a sunny day and I thought I’d try to get them to work. They darkened quite nicely as soon as I removed them from their case.
What kind of light affects Transition lenses?
Do modern car windshields have an ingredient or coating, such as a UV blocker, that blocks the kind of light needed to darken the lenses?
The reaction is Light + 2Ag + 2Cl <==> 2Ag + Cl[sub]2[/sub]. The more silver there is, the darker the lenses become. But you start out with 2Ag and end up with 2Ag, so I don’t seen an increase in the amount of silver. (Wouldn’t it be great if you could just create silver from light? ) Is it because the chlorine becomes dissociated from the AgCl, leaving relatively more silver in the mix?
2 AgCl <–> 2Ag + Cl[sub]2[/sub]. AgCl is the formula for a salt. To indicate it’s part of the crystaline structure of the glass and not a salt all on it’s own (I don’t know if this is the case) you could write, 2Ag[sup]+[/sup] + 2Cl[sup]-[/sup], but not 2Ag + 2Cl
When exposed to UV light the silver ions turn into silver metal, with dramatically different properties.
Yes this sucks but there doesn’t seem to be a present way around it (at least not that my optician knew about, because I asked specifically). I have to have seperate prescription sunglasses for driving.
When asked this question, my optometrist** said most transition lenses sense* the light level more at the edges (usually the top edge) than on the face of the lens. He pointed out that the majority have pretty thin or non-existent frames holding the lens, in order to let light in around the lens perimeter. According to him, a car’s roof is really effective at reducing overhead light that would normally reach the top of your glasses’ lenses, therefore they don’t darken as much.
The solution is elegant and simple Johnny, you need a convertible.
*my wording; I don’t know the science behind this.
**grain of salt, etc. We were well into a twelve-pack at this point, and he’s been known to pull my leg.