Yellowing frosted glass lights in garden

Dilly

Hi
I have 3 garden lights which over time have turned yellow. I think this could be because of UV rays.
Originally the glass shades were frosted white. However we had some work done in the garden and the lights were removed. It was then that I discovered the discoloration. I'm not 100% sure if they are glass or some kind of acrylic. You can now see where the bars from the frame sat, as this is still the original colour
Does anyone know how I can get the glass back to being white or even whiter?
Thanks for any suggestions

chrisw

Glass doesnt go yellow, or if it gets stained then it will wipe off.

Therefore these are likely to be some form of plastic or acrylic and, if it's not dirt that will scrub off, will have deteriorated due to the UV from the sun. If so, they can't be rewhitened, only replaced.