How does Jack's attack on Piggy and the breaking of one of the lenses in his spectacles symbolize the degeneration of the group in chapter 5?
The group that assembles at the blowing of the conch represents organized society. The argument during this late assembly in chapter 5 is about ghosts and beasts. Piggy believes in the assembly, believes in rules and believes in an organized society. He does not believe in ghosts or beasts. As he makes clear:
“The trouble is: Are there ghosts, Piggy? Or beasts?”
“Course there aren’t.”
“’Cos things wouldn’t make sense. Houses an’ streets, an’—TV—they wouldn’t work.”
What Piggy is saying is that there is a rational world, a world that is based on and works because of laws of nature and scientific inquiry carefully applied and rules of construction and order. Things wouldn't work and wouldn't make sense if chaos and superstition reigned.
Jack represents disorder, violence and chaos as opposed to Piggy's common sense, intellectuality and sense of truth and order. And his glasses are the symbol and the embodiment of who he is and what he stands for. They are a man-made construction based on observation and applied technology for the purpose of seeing the world clearly. And when the glasses are broken, this betokens the disintegration, the breaking, of the order that is the civilized assembly of the group.