Observations
When light fell onto a metal plate it released electrons from the surface straight away. Increasing the intensity increased the number of electrons emitted. If the frequency of the light was lowered, no electrons were emitted at all. Increasing the intensity and giving it more time did nothing, no electrons were emitted.
If Light was a Wave…
Increasing the intensity would increase the energy of the light. The energy from the light would be evenly spread over the metal and each electron would be given a small amount of energy. Eventually the electron would have enough energy to be removed from the metal.
Photon
Max Planck had the idea that light could be released in ‘chunks’ or packets of energy. Einstein named these wave-packets photons. The energy carried by a photon is given by the equation:
Explaining the Photoelectric Effect
Einstein suggested that one photon collides with one electron in the metal, giving it enough energy to be removed from the metal and then fly off somewhere. Some of the energy of the photon is used to break the bonds holding the electron in the metal and the rest of the energy is used by the electron to move away (kinetic energy). He represented this with the equation:
hf represents the energy of the photon, f is the work function and EK is the kinetic energy.
Work Function, f
The work function is the amount of energy the electron requires to be completely removed from the surface of the metal. This is the energy just to remove it, not to move away.
Threshold Frequency, f0
The threshold frequency is the minimum frequency that would release an electron from the surface of a metal, any less and nothing will happen.
Since , the minimum frequency releases an electron that is not moving, so EK = 0
which can be rearranged to give:
Increasing the intensity increases the number of photons the light sources gives out each second.
If the photon has less energy than the work function an electron can not be removed. Increasing the intensity just sends out more photons, all of which would still not have enough energy to release an electron.
Nightclub Analogy
We can think of the photoelectric effect in terms of a full nightclub; let the people going into the club represent the photons, the people leaving the club represent the electrons and money represent the energy.
The club is full so it is one in and one out. The work function equals the entrance fee and is £5:
If you have £3 you don’t have enough to get in so noone is kicked out.
If 50 people arrive with £3 no one has enough, so one gets in and noone is kicked out.
If you have £5 you have enough to get in so someone is kicked out, but you have no money for booze.
If 50 people arrive with £5 you all get in so 50 people are kicked out, but you have no money for booze.
If you have £20 you have enough to get in so someone is kicked out and you have £15 to spend on booze.
If 50 people arrive with £20 you all get in so 50 people are kicked out and you have £15 each to spend on booze.