Photoelectric effect

1887 – Heinrich Hertz discovered when light of a certain frequency on a metallic surface, electrons are emitted

If the light does not have a high enough frequency, no electrons are emitted no matter the intensity.

  • The electrons were emitted immediately
  • Increasing the intensity of the light increased the number of emitted electrons but not their maximum of KE
  • Low frequency did not cause the ejection of electrons, no matter what the intensity of the light
  • A weak high frequency will eject only a few electrons, but their maximum KE are greater than those for intense light of lower frequencies

Particle model

The energy of the ejected electrons was:

  • Proportional to the frequency of the illuminating light
  • Independent of the intensity of the light

Light therefore acted as a particle, and does not act as a wave in this instance.

If it were a wave, the energy of the electrons would depend on the amplitude/intensity, not the frequency.

Work function

W = the minimum amount of energy required to release an electron from a metal.

W = hfo

Where fo is the minimum frequency needed to eject electrons, h = Planck’s constant

EKE max = 0.5 mv2 = hf – W

The KE of the electron is equal to the energy of the photon minus the amount of energy needed to remove it from the atom

 Stopping voltage

Light on the negative plate (cathode)
The ejected electrons will move towards the positive plate.

The energy that it gains as it moves across a field of potential difference is given by:

E = qv = eV

That is, the energy that an electron gains across an electric field of a pd of 1 V is one eV.

Light on the positive plate (anode)

  • Electrons will be ejected from the +ve plate and towards the -ve plate
  • However, the electric field will force the electrons back towards the +ve plate
  • The voltage that just stops the fastest electrons from hitting the -ve plate is called the stopping voltage
  • The energy that the field removes from the electron is equal to their KE

eVo = KE = hf – W

Where e = charge on electron; Vo = stopping voltage



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