the following simulation maybe an inspiration?!?!
http://dl.dropbox.com/u/3579325/pencil%20and%20screen.jar

Actually the best way is to do an experiment. However i do not have the resources. I am currently self-studying physics. My understanding is basic and i cannot really visualise anything:-[ . I hope some pros can help newbies visualise without constructing an experiment, be it a simulation or animation or youtube video of the experiment. thx a lot.

PS: if not for the free simulations at this site i could not have got this far :-\ keep it up.