Quantum Phenomena

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3.1 Photoelectricity

  • A metal contains conduction electrons, which move about freely inside the metal.
  • The electrons collide with eachother and with the ions of the metal.
  • Electrons are emitted from the surface of a metal when electromagnetic radiation above a certain frequency is directed at the metal.
  • This is called the photoelectric effect.
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3.1 Photoelectricity (cont)

  • The incident EM radiation must be at least at a certain frequency, known as the threshold frequency, for photoelectric emission to occur.
  • The threshold frequency depends on the metal.
  • The wavelength of the incident EM radiation must therefore be below a maximum wavelength value for photoelectric emission to happen.
  • If the radiation is below the threshold frequency, no emission will occur, no matter how high the intensity.
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3.1 Photoelectricity (cont)

  • Light is composed of photons, each with energy=hxf.
  • When light is incident on a metal surface, 1 electron absorbs 1 photon and gains all its energy (=hf).
  • An electron can leave the metal surface if the energy it gains exceeds the work function of the metal.
  • The work function is therefore the minimum energy needed by an electron to escape from the metal surface.
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3.1 Photoelectricity (cont)

  • So, an emitted electron will have maximum kinetic energy when it has absorbed energy =hf, and lost just the work function of the metal, and nothing else (eg by collision).
  • Max kinetic energy= hf - work function
  • The minimum energy is just the work function, so:
  • work function= h x threshold frequency.
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3.2 More about Photoelectricity

  • If an electron absorbs photon energy=hf, but that does not at least equal the work function of the metal, the electron will collide repeatedly with other electrons or metal ions until it loses its extra kinetic energy. 
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3.2 More about Photoelectricity (cont)

  • A vacuum photocell is a glass tube that contains a metal plate (photocathode), and a smaller metal electrode (anode).
  • When light greater than the threshold frequency is directed at the photocathode, electrons are emitted from the cathode which travel to the anode.
  • When the vacuum photocell is in a circuit with a microammeter, the photoelectric current can be measured.
  • The photoelectric current is proportional to the number of electrons transfered per second. 
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3.2 More about Photoelectricity (cont)

  • When you plot max kinetic energy of a photoelectron against frequency of the incident light, you get a linear y = mx + c graph.
  • y= max kinetic energy
  • m= plancks constant (h)
  • x= incident light frequency
  • c= the negative version of the work function.
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3.2 More about Photoelectricity (cont)

   (http://www.antonine-education.co.uk/Image_library/Physics_5_Options/Turning_points/graph1_4.gif)

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3.3 Collisions of Electrons with Atoms

  • An ion is a charged atom, due to the atom losing or gaining electrons.
  • The removal or addition of electrons to an atom is called ionisation. 
  • The electron volt is a unit of energy equal to the work done when an electron is moved through a potential difference of one volt. 
  • To convert electron volts (eV) into joules, multiply the eV number by the charge of an electron. 
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3.3 Collisions of Electrons with Atoms (cont)

  • When a free electron collides with an electron orbiting an atom with enough energy, the atomic electron absorbs all the colliding electron's kinetic energy.
  • With this energy, the atomic electron moves up to a higher energy level in the atom.
  • This is called excitation. 
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3.3 Collisions of Electrons with Atoms (cont)

   (http://www.emc.maricopa.edu/faculty/farabee/biobk/excitation.gif)

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3.4 Energy Levels in Atoms

  • Electrons are held together in shells of an atom because of the electrostatic force of attraction they have with the nucleus.
  • The energy of an electron in a shell is constant, and electrons in shells further from the nucleus have more energy than those closer to the nucleus.
  • The lowest energy state of an atom is its ground state.
  • When a ground state atom absorbs energy, one of its electrons moves to a higher energy level. The atom is now in an excited state. 
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3.4 Energy Levels in Atoms (cont)

  • The electron configuration of an excited atom is unstable because there is now a vacancy in the shell the electron moved from.
  • An electron therefore falls to the vacant lower energy level from a higher energy level, moving the atom to a lower energy state. 
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3.4 Energy Levels in Atoms (cont)

  • As this electron de-excites, it emits a photon.
  • The energy of this photon is equal to the energy lost by the electron and therefore the atom. 
  • An excited atom may de-excite either directly or indirectly.
  •  If the excited atom is in its second excited state, it may drop to the first excited state, then the ground state (emitting 2 photons of different energies).
  • Or, the atom can directly de-excite to the ground state (emitting 1 photon of energy = sum of the 2 indirect photons.)
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3.4 Energy Levels in Atoms (cont)

  • When an electron absorbs an incoming photon, the electron gains all the photon's energy.
  • If this energy is exactly equal to the energy required to move the electron up an energy level, the photon will be absorbed, and the electron will be excited. 
  • If this energy is larger or smaller than the energy required, the photon will not be absorbed by the electron. 
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3.4 Energy Levels in Atoms (cont)

  • The fluorescent tube is a glass tube with a fluorescent inner coating.
  • The tube contains mercury vapour at lower pressure. 
  • When a current is applied through the tube, mercury atoms collide with eachother and other tube electrons.
  • This excites the mercury atoms. The mercury atoms emit both UV and visible photons when they de-excite.
  • The UV photons are absorbed by atoms in the fluorescent coating, causing more excitation.
  • The coating atoms de-excite and emit more visible photons.
  • The combination of visible photons makes the tube fluoresce. 
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3.5 Energy Levels and Spectra

  • The wavelengths of the lines in a line spectrum of an element are unique to the atoms of that element.
  • By measuring the wavelengths of a line spectrum, we can identify the element that produced the light.
  • The different coloured lines are due to groups of photons with different energies, and different wavelengths as a result (all photons in a group have the same energy). 
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3.5 Energy Levels and Spectra (cont)

  • Helium was discovered from the spectrum of sunlight.
  • A pattern of lines in the spectrum was observed at wavelengths that had not been observed with any other known gas.
  • The pattern must therefore have been from a previously unknown element in the sun- Helium. 
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Wave Particle Duality

  • Light has a dual nature, in that is can behave as a wave or a particle.
  • The wave-like nature of light is observed when the diffraction of light takes place (eg light passing through a slit).
  • The particle-like nature of light is observed in the photoelectric effect. 
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Wave Particle Duality (cont)

  • Particles of matter also have a dual wave-particle nature.
  • The particle-like nature of matter is observed when electrons are deflected by a magnetic field.
  • The wave-like nature of particles is observed when electrons at a constant speed are diffracted through thin metal foil, to produce a pattern of rings.
  • This wave-like nature of a particle is characterised by its de broglie wavelength. 
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