A beam of photon of energy 5.0 eV falls on a free metal surface of work function 3.0 eV . As soon as the photoelectrons are emitted, they are removed. But the emission comes to a stop after a while - explain the reasons.

Explanation

When the photons of energy 5.0 eV fall on a free metal surface of work function 3.0 eV, electrons are emitted from the surface instantaneously and the surface becomes positively charged. After a while, the number of free electrons in the metal surface decreases and more energy is required for emission of electrons from the metal surface, i.e., the work function of the metal increases. When the work function becomes greater than 5.0 eV, the photons of energy 5.0 eV are not able to efect electrons from the metal surface. So, the emission of electrons ceases after some time. To overcome this difficulty, the metal surface is connected to a negative pole of an electric source to make it negatively charged.