As we know that convex lens focuses the image and in case of Sun can even burn a paper. Then while wearing the lenses or spectacles and looking towards the Sun why our eyes(retina)don`t get damaged?

@ kartik 
paper are so easy to burn , but our i mean human eye are made up of complex tissue and organ system , and it cannot be burnt so easily . 
and you are talking abt damage – yes , that happens , if u will stare the sun constantly for 3-4 minute , your eye will autometically close down while doing so , its a dengerious experiment and dnt try to do that , 
and the paper will get burnt after 5-8 muinute , not immidiately . 
HOPE IT CLEARS NOW