In an otherwise empty room, Zendaya is standing with her back against the north wall and looking south. Elara is standing with her back against the south wall and looking north. A tall bookshelf is placed directly between them, blocking their view of each other. If Elara starts moving along the wall toward the east corner, at what point will Zendaya be able to see her?

Respond very briefly.
Zendaya will be able to see Elara when Elara moves enough to the east so that her line of sight with Zendaya clears the eastern edge of the bookshelf. This will occur once Elara reaches a position where a straight line can be drawn from her eyes to Zendaya's eyes without intersecting the bookshelf. The bookshelf ceases to be an obstacle to their view when there is no longer any part of it between the direct line of sight from Elara to Zendaya.