Forum:KB:Why didn't Luke know about lightsabers?

In Obi-Wan Kenobi's house, right after he introduces himself to Luke Skywalker, why doesn't Luke know what a lightsaber is? Lightsabers and the Force were important parts of his galaxy's history, and he seems cluelesss about it all. Even his slave father who lived on the same planet knew about Jedi and lightsabers as a child.