The prism is a mirror that lets you see the screen. But you're focusing entirely in the wrong part of this. The important thing is that it's a small display off in the corner, not blocking or overlapping what you're looking at. It's not aiming a light at your eye any more than a book would.

http://www.catwig.com/google-glass-teardown/teardown/display...

The 'screen' is projected into your eye. The mirror is a lens that helps collimate the light for this purpose. The attentional split of the device is a different argument, but I was just correcting this error that there's 'nothing projected into the eye'. If your fundamentals are wrong, chances are you aren't aware of the caveats in this field.