Hi, I've created a laser beam like shown in this video: http://www.youtube.com/watch?v=E1erYLfqk80&NR=1

now I want it to end when it hits an object, like it would in reality, so how do I achieve that? like this: http://www.youtube.com/watch?v=1mF8JKwzxMI

thanks in advance
havoc