SuperJoost23

I have a big assignment for school, in which I will be researching wether or not it is dangerous or not to wear cheap sunglasses, as you have your eyes more open when there is less visible light, if UV was to come through cheap glasses it could be dangerous. Now I do understand why some wavelengths of UV have a higher penetration power, however I don't understand what defines what material is good at letting UV through or blocking it. Is it just density? Does is have something to do with the way molecules are arranged? How could I find what is a good UV blocker, and why it is?