When light of suitable frequency falls on a metal surface, electrons are emitted from them. This effect is known as the photoelectric effect.

Because the work function is the minimum energy required. Some electrons will need more than this minimum amount to escape the metal.

E=hf, h-plancks constant, f-frequency of light

It is the minimum potential difference required to stop the electron. (To remove all of its Kinetic energy)

Because the electron uses some energy to remove itself from the metal. KE=hf-Ф

The work function is the minimum amount of energy required to remove an electron from a metal.

We use electron volts because if we use Joules, the quantities will be very small.

maximum possible value of kinetic energy of electron is 9eV

the lowest frequency of electromagnetic radiation that will result in the emission of electrons from a specified metal surface.