PHYA1 - Quantum

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  • Created by: Franklin
  • Created on: 27-04-14 10:05

What is the photoelectric effect?

When radiation of a high enough frequency is incident upon a metal surface photoelectrons will be emitted from the surface.

The puzzling problems

  • The observations made from the photoelectric effect caused confusion as it didn't follow the wave theory of EM radiation
  • Photoelectric emission does not take place if the incident radiation is below the threshold frequency of the metal
  • The no. of photoelectrons emitted per second from the surface of the metal is proportional to the intensity of the radiation since one electron absorbs one photon

The wave theory cannot explain the photoelectric effect which is why Planck and Einstein had to come up with a new one

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Einstein's Explanation

In 1905, Einstein put foward his photon theory of light. saying that they can only exist in discrete packs of energy, quanta

  • Energy of a photon = hf (Planck's constant 6.63 x 10-34 
  • If from the photon the electron recieves enough energy to overcome the work function of the metal, it will escape the metal surface as a photoelectron
  • The work function is a property of the metal

photolectric effect equation (

Why varying Ek of photoelectrons?

  • Energy of photon is constant/fixed OR energy given to electron is fixed
  • Energy required for electron to leave/escape/emit from the surface/metal OR electron has to overcome work function
  • Maximum kinetic energy is the energy of photon minus the work function
  • Deeper electrons require more energy to get to the surface
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Photoelectric Equation

photolectric effect equation (

  • hf = Energy of incident photons
  • ( function - the minimum energy required to remove an electron from the surface of the metal
  • Ek = maximum kinetic energy of photoelectrons

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Graph of max K.E against f

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Collisions of electrons with atoms

Ions are atoms with an overall charge and so no. of protons does not equal the number of electrons

  • Alpha, beta and gamma radiation create ions when they pass through a substance and collide with the atoms of a substance

The electron volt is a unit of energy = 1.6 x 10-19  . The work done by an electron per when when the current is 1A

- Using a gas-filled tube with a metal filament we can show that gaseous atoms can absorb energy without being ionised, this is called excitation

  • The lowest energy state of an atom is called the ground state
  • When an atom in its ground state absorbs energy it becomes excited
  • The difference in energy between the energy levels must equal the energy of the incoming photon otherwise it will not be absorbed by the electron
  • A photon is released by an electron when it drops to a lower energy level, energy of photon = change in energy level
  • An excited atom can de-excite directly or indirectly to the ground state
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  • Ionsiation and excitation occurs when mercury atoms collide with each other and with electrons emitted from the electrodes
  • The mercury is gaseous as it needs to be a certain distance from the electrodes 
  • the mercury atoms then emit UV photons
  • The UV photons are absorbed by the phosphorus coating, which excites the phosphorus atoms
  • The atoms de-excite and emit  photons of a range of frequencies lower wavelength
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Wave-particle duality

Wave-like nature is observed in the diffraction of electrons

Particle-like nature of waves is observed in the photoelectric effect

De-Broglie Wavelength

  • matter particles have wave-partcle duality
  • Wave-like bahaviour of a matter particle depends on its momentum, p (mv)


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