How hard do u have 2 hit your head to lose ur eye site?

Question

How hard do u have 2 hit your head to lose ur eye site?

Answer 1

You don't.  You just need to be unlucky.

The eyes may survive extreme levels of trauma, or very small injuries just at the wrong time or place may have disastrous results.

Sitting down in a chair heavily might be enough to start a retinal detachment, if the resulting pressure wave happened to flow "just right."  

Answer 2

uhh. really hard! and you don't want to lose your eye sight!