UV, Glass, and Glasses

They claim this is why you cannot get a suntan in a car with the windows up, but I don’t totally get it! In days of yore, back in physics class, it was demonstrated to us how UV does not pass through glass. Well, now years later backed with years of experience, I now know the proper questions I should have asked back then!

The demo involved shining a UV light at some oblique angle onto the pane of glass. The result, as I recall, was an interference pattern. Allegedly, the teacher said that this proves the UV does not pass through the glass. The explanation was that the UV is bouncing of the inside of the back surface thus causing interference with the UV incident on the top surface. But, I don’t get it!

a) Why would the UV pass through the top surface and not the back surface?

b) What the UV lightwaves were incident normal (perpendicular) to the glass surface? Would it pass straight thorugh?

c) I have to wonder if the UV coatings “push” from eye docs might not just be taking advantage of a propery inherent to the eyeglasses anyhow! Does anyone know if there is a relation between index of refraction and the transmission of UV through a medium???

a) You are perfectly correct. If glass was opaque to UV (and it is, for most of the UV band), the UV light would not reach the far surface, and there would be no interference pattern. Actually, even if you use visible light, to which the glass is transparent, you still wouldn’t get an interference pattern because you need much better optical surfaces than common plate glass.

b) Glass is fairly opaque to UV even at normal incidence.

c) You are correct as far as glass lenses are concerned. However, most eyeglasses today are made from plastic, which is transparent to UV. Here, anti-UV coatings are useful. It is especially important for sunglasses which block visible light and makes the pupils open up, admitting a lot more UV than if you weren’t wearing the sunglasses.

The last time I got a new pair of glasses I asked them about UV coatings, and they correctly told me that it was useful for plastic lenses but not necessary for glass lenses. So I don’t think they are taking unfair advantage of people’s ignorance. On the other hand, some car manufacturers advertise anti-UV coatings on windshields - that seems bogus to me, as nobody makes plastic car windows. (Except rear windows on convertibles)

Personally I haven’t a clue but I recently heard on TV exactly the opposite: that glass lets more UV through and plastic lenses are better. I was going by that…

Not entirely on topic, but here’s Cecil Adams on UV and glass.