In a rectangular room, Marcellus and Lavinia are standing at opposite corners. A large bookshelf, as tall as the room, is placed against one of the walls, obstructing the direct line of sight between the two corners. If Marcellus ties a balloon to a string and lets it float upwards, can Lavinia see the balloon as it goes up?

Respond very briefly.
Lavinia cannot see the balloon directly as it ascends because the line of sight from Lavinia to the balloon is blocked by the bookshelf. However, as the balloon rises above the height of the bookshelf, if there is a gap between the top of the bookshelf and the ceiling, Lavinia will be able to see the balloon upon its emergence above the bookshelf. If the bookshelf is flush with the ceiling, the balloon remains out of Lavinia's sight unless its path moves beyond the edge of the bookshelf.