Quantum physics

• Photons:

  • Electromagnetic radiation had a particular nature - it was tiny packets of energy, rather than a continuous wave
  • The energy of each photon is directly proportional to its frequency
  • E = hf where E is the energy of the photon in J, f is the frequency of the electromagnetic radiation in Hz, and h is the Planck's constant (6.63 x 10^-34)
• • E = hc / lambda
  • The energy of a photon is inversely proportional to its wavelength
  • Short wavelength photons, like x-rays, have much more energy than long wavelength radio waves
• Electronvolts (eV) --> The energy of 1eV is defined as the energy transferred to or from an electron when it moves through a potential difference of 1V

  • W = QV = eV
• LEDs & Planck's constant:

  • LEDs convert electrical energy into light energy
  • The emit visible photons when the pd across them is above a critical value (the threshold pd)
  • When the pd reaches the threshold pd the LED lights up & starts emitting photons of a specific wavelength
  • At this pd the work done is given by W=QV
  • This energy is about the same as the energy of the emitted photon.
  • We can use the voltmeter to measure the minimum pd that is required to turn on the LED.
  • A black tube placed over the LED helps to show exactly where the LED lights up.
• • At the threshold pd, the energy transferred by an electron in the LED is approximately equal to the energy of the single photon it emits
  • threshold pd x charge on electron = energy of emitted photon
  • Ve = hf
  • Ev = hc/ f
• Photoelectric effect —> the ejection of electrons from the surface of a metal when it is exposed to electromagnetic radiation of sufficiently high frequency (or short wavelength)
• Laws of photoelectric effect:

  • Increasing the intensity of the radiation incident on a metal surface increases the number of electrons emitted per second
  • If the incident radiation frequency(f) is a certain threshold frequency(f0), no electrons are emitted, no matter how intense the radiation is
  • Different metals have different (f0) & (lambda 0) values
• • The photoelectrons are emitted from a given metal with a range of kinetic energies , from zero up to a maximum value
  • The maximum kinetic energy (KE max) of the emitted electrons increases with the frequency of the incident radiation and it is independent of the intensity of the radiation
• threshold frequency —> the minimum frequency of electromagnetic radiation which will cause photoelectric emission
• threshold wavelength —> the maximum wavelength of electromagnetic radiation which will cause photoelectric emission
• • Kinetic energy gained by electron = energy of incident photon
  • 1/2mv^2 = hf
• Work function —> the minimum energy needed by an electron in order to escape from the metal surface
• Photoelectric equation:

  • When a photon of energy (hf) causes photoemission from a metal surface, some of the photon energy is used to overcome the work function, while the remainder appears as kinetic energy of the emitted electron.
• hf = work function + KE max
• • incident photon energy hf0= work function
• Observations of the gold leaf experiment:

  • placing the UV light source closer to the metal plate causes the gold leaf to fall more quickly
  • Using a higher frequency light source doesn’t change how quickly the gold leaf falls
  • using a filament lamp source causes no change in the gold leafs position
  • using a positively charged plate causes no change in the gold leaf’s position
  • Emission of photoelectrons happen as soon as the radiation is incident on the surface of the metal
• Interaction between a photon and surface electron


  • Each surface electron can only interact with a single photon
• This provides important evidence that light is quantised or carried in discrete packets
• The number of photo electrons emitted is exactly equal to the number of photons incident the surface in which the photoelectric effect is taking place
• Increasing the intensity of the electromagnetic radiation

  Increases the number of photos per area incident on the surface
• Increasing the number of photons per area incident on the surface


  Increases the number of photoelectons emitted from the surface
• Wave - particle duality --> A model used to describe how all matter has both wave & particle properties
• Electron diffraction:

  • Electrons have a mass & charge so they can be accelerated & deflected by electric & magnetic fields
  • Under certain conditions electrons can diffract. They spread out like waves as they pass through a tiny gap, and can form diffraction patterns in the same way as light
  • For diffraction to occur the size of the gap through which the electrons pass must be similar to their wavelength
• Wave - particle duality:

  • The wavelength of a particle is inversely proportional to its momentum
  • lambda = h / P --> de Broglie's equation
  • h = Planck's constant