Explain how:
The bombarding electrons cause the atoms of the vapour to emit photons.
The existence of a spectrum consisting of lines of a definite frequency supports the view that atoms have discrete energy levels.
Bombarding electrons collide with atoms of the vapour, transferring its energy and so the atoms become excited. This makes the electrons move to a higher energy level. Electrons in the excited state are unstable and so cascade back to the ground state(de-excitation). During this process the electrons lose energy and emit a photon at the same time.
The energy of the photon is equal to the difference in energy levels. The frequency of the emitted photon is given by E1-E2=hf. Atoms have discrete energy levels and since only certain photon energies are allowed, you only see the corresponding wavelengths in the line spectrum.
(something like this will get you 5-6 marks)
Do you know why the kinetic energy of an emitted electron has a maximum value?
The electrons in the metal all receive the same energy from the incident photons.(E=hf)
But the work done by the electron to get the the surface may be different.
if you look:
Ekmax = hf - (workfunction)
h is always the same
f is the same
so workfunction must be different