Question

What causes emmision of electron from the surface of the metal​

Answer 1

Explanation:

Electron emission is the process when an electron escapes from a metal surface. Every atom has a positively charged nuclear part and negatively charged electrons around it. Sometimes these electrons are loosely bound to the nucleus. Hence, a little push or tap sets these electrons flying out of their orbits

Answer 2

When light of a certain threshold frequency strikes on a metallic surface,it causes emission of electrons from the surface of the metal.

These phenomenon is called photo electric effect.