Questions & Answers about La luz del fondo de la sala está encendida.
Both are possible, but they mean slightly different things:
del fondo de la sala: literally “of the back of the room”, focusing on which light it is (the one that belongs to / is located in the back area).
- la luz del fondo de la sala = the light (that is) at the back of the room.
al fondo de la sala: means “at the back of the room” and usually describes where something is, not which one.
- La luz está al fondo de la sala = The light is at the back of the room.
In your sentence, del is used because it’s identifying which light (the one from the back), not just giving a location.