Blocking a light source

This is probably a really dumb question, but here goes anyway.

I notice that if you place a solid object, like a wall, between a light source and another object, like a screen, the screen lights up as if the wall were transparent. Light from one room bleeds right through to another room.

Is there any way to block a light source? Is it dependent on the user's graphics card?

Nope. That would take ray-casting, and I don't think anyone is running on Beowulf clusters...