Many Years ago all headlights were made of glass. Then came along Health and Safety, and decided it was best to have headlights made of hardened plastic. The majority of these plastic headlights also have a clear coat protection put on them.
You may have noticed that most cars over 10 years old are starting or completly turning yellow, which means it becomes harder to see while driving in the dark. This also meams your lights could fail an MOT if the have gone to the extreme.
Why does this happen? due to exterior eliments, sun, rain, snow, trees, salt, road dirt. I have also noticed that some of these car wash pop up stations cover your vehicle with Traffic Film remover (TFR). all this will turn your lights from brilliant clear to an yellow substance. Of course this can be treated and protected But you can protect before this happens.
To Help Prevent all you need to do is make sure when you wash and polish your car you do the same with your lights, with a good high end wax.