What happens to the symbols of the conch and Piggy's glasses and their meanings?
Originally, the conch stands for order, reason, and organization. Ralph uses the conch to summon the boys. When they vote for chief, one of the boys suggests Ralph. Ralph has leadership qualities but holding the conch, he also seems to be in command. The boys continue to use the conch to organize meetings and establish order. Over time, Jack lures some of the boys to his camp. Ralph's camp diminishes and order fades as well. The conch loses its power to organize the boys because the majority of the boys devolve into Jack's savage followers. In Chapter 12, when the conch is smashed with Piggy's death, the presence of order and civilization is killed as well. It is a symbolic end to any notion of order and reason.
Piggy's glasses are a symbol of technology and therefore, civilization. The boys use his glasses to start the fire. The fire is the means to attract the attention of a ship. Therefore, Piggy's glasses are used to hopefully connect the boys back to civilization. So, the glasses are associated with technology and are a link to civilization. When one of Piggy's lenses is broken, he is basically half-blind. This parallels the boys' descent into savagery. The most intelligent, civilized boy becomes half-blind. This illustrates how the boys begin losing their civilized ways. The start losing the ability to "see" the importance of order and reason.
The glasses are also destructive. Using them to ignite the fire, the boys nearly set the entire island on fire. So, technology and civilization are not completely infallible in this novel. Note that while the boys are on the island, an atomic war is going on. In that "civilized" world, technology is used for destructive purposes. Likewise, the glasses are capable of destruction. Golding raises the question of who is more or less civilized: the savage boys with glasses or the civilized adults waging war with atomic weapons.