S.O.S. Mathematics CyberBoard

The kinetic energy with which an electron from a metal surface is emitted is dependant on the frequency of the incident radiation rather than the intensity. So far, the only proof for this statement I have come across is "experimental results show it".
My question more specifically is:
Why is there a threshold frequency? I am prepared and would be more grateful if I had the chance to do some research on my part so if anyone can mention experiments or theories, I'll look them up.
Thank you.
Regards,
Joseph

Further to my earlier post, Einstein's Photoelectric Equation begins to answer my question superficially. Since I'm on my last year in school, my textbooks don't explain how Einstein came up with his equation...all they say is based on experimental evidence. I tried searching the net, however since the internet is such a vast source in info (which can be a good and a bad thing) I'm not really getting the information I need. I would be really grateful if I could be given a tiny push to guide me in the right direction in finding why is the K.E. of emitted electrons is only based on frequency. Thanks again.
Regards,
Joseph