Why do prescription glasses that darken in sunlight fail to darken when the light travels through a car windshield? I was going to buy this add on, as I almost exclusively use my sunglasses for driving. I'm just stacking them over my normal ones now... but it's quite annoying that I almost paid the extra $$$ for this feature. Found out at the last second, luckily.
 A: Reaction sunglasses (also known as photochromic glasses) go dark in reaction to ultraviolet light with wavelengths from 280-320 nm. The smallest wavelength of visible light is around 390 nm. Ordinary glass blocks over 90% of light with wavelength below 300 nm but allows longer wavelength UV to pass. The glass used in car windows is often treated to block more UV light. This is to protect the interior of the car from light induced ageing but it also helps
drivers avoid sunburn. This means that most of the UV light that causes the glasses to react is filtered out by the car windows, but not all. 
Another important factor is that the most intense UV light that falls on the glasses when you stand outside is coming directly from the Sun at a high angle, not from the direction you are looking in. In a car this is usually blocked by the roof of the car or a sun visor. UV light is mostly absorbed by any surface it falls on so the ambient UV light inside a car would be quite low even if the windows were open
A: These type of glass/plastic responds to UV light. Car windshield has UV filters so the glasses does not darken as much inside cars.
A: These glasses probably respond to the amount of ambient light. In a car the ambient light is substantially less than outside. Your eyes are focused on the wind shield, but glasses are also affected by the dark dashboard, ceiling, etc 
