A question I've had for a while now

[Mark Collins]

So, sorry if this is a noob question but it's been bothering me for a while.

Basically, I'm wondering what physical part of a lens is measured to get the focal length.

For example, on a 50mm lens, how would they arrive at saying that the lens is a 50mm?

[David Mullen ASC]

http://en.wikipedia.org/wiki/Focal_length

[Mark Collins]

http://en.wikipedia.org/wiki/Focal_length

Thank you good sir