All polycarbonate (plastic) lenses will go dull and yellow after sometime. You can either polish them yourself (buy a polishing kit, as below) or go to any car detailing shop (CarMaxx etc) and they'll do it for you.

Here's a video guide:

http://www.youtube.com/watch?v=BETIgHjMrpw

You can buy Meguiars PlastX in Carrefour.

Guide:

http://www.popularmechanics.com/cars/how-to/maintenance/4252611