The Photoelectric Effect

Photoelectric Effect

If EM Radiation is incident on a metal surface, electrons may be emitted. The electrons emitted are from the so-called sea of delocalised electrons (not electrons bound to any particular atom).

Several features of the results observed cannot be explained using the traditional wave model of light:

For each different metal, there exists a threshold frequency, f0, below which NO EMMISSION occurs no matter how high the INTENSITY of the radiation.

Above the threshold frequency, emission occurs instantaneously.

Maximum Kinetic Energy of emitted electrons (photoelectrons) depends ONLY on frequency of the incident radiation (not on intensity).

According to the wave model:

Emission should occur at ANY frequency.

o Since waves continuously transfer energy, electrons should gradually absorb enough energy to escape.

Emission should not be instantaneous since wave has to gradually transfer energy to electron.

Intensity of incident radiation should also affect Max KE of emitted electrons since higher intensity of radiation means greater power per unit area incident on the metal (i.e. electrons may gain more energy)

The observed results are better explained by a quantum or photon model of radiation.

Light and other EM radiation is quantised into parcels or packets of energy called photons.

Photon energy E = hf

Each different metal has a different Work Function, & the minimum energy required to emit an electron.

EACH ELECTRON CAN ABSORB ONLY ONE PHOTON AT ANY ONE TIME.

If the frequency of radiation is below the threshold frequency, each photon will have insufficient energy to release an electron from the metal.

I.e. & = hf0

Above the threshold frequency, maximum KE of emitted electrons is the extra energy provided by each photon above the work function.

o KEmax = hf

o Electrons below the surface of the metal require energy greater than the work function to escape so are emitted with lower kinetic energy.

Photons are absorbed instantly so electron emitted instantly.

Increasing intensity means more photons (per second, per m2) so results in more electrons emitted (per second) but no change to max KE.

Increasing frequency means more energy per photon so higher max KE of electrons but same rate of electron emission.

Stopping potential

With vacuum photocell (fig 1)

o UV radiation incident on metal plate (cathode)

o Electrons cross gap to anode

o Micro-ammeter measures current flowing in circuit.

By using the variable power supply to increase positive potential on cathode, current can be reduced to zero

o Electrons are attracted back to positive potential on cathode so none cross the gap to the anode.

Stopping potential is supply voltage at point where current in circuit drops to zero.

At this point WORK DONE BY POWER SUPPLY = MAX KE OF ELECTRONS

o e x Vs = hf