Can mobs see through iron bars?
Besides, what blocks can mobs see through?
Most mobs cannot see through most solid blocks, including semi-transparent blocks such as ice, glass, tall grass, or glass panes. Mobs do not attempt to walk on rail tracks, unless pushed on by other mobs.
Also, can mobs see through trapdoors?
Mobs now recognize open trapdoors(except for oak trapdoors) as holes! I've noticed that, in snapshot pre 4, mobs don't fall in holes made by open trapdoors. For now, there's only one solution: use oak trapdoors. They still work just as before.
Since iron bars can not be destroyed by a Ghast's fireball, and since they block a ghast's line of sight, iron bars can be used as "Ghast-proof" walls for either a base, or for small enclosed roads through the Nether. Iron bars can prevent fire from spreading.