Quantum Physics

• What is a use of line spectra?

  • Forensic scientists can use line spectra from samples collected at a crime scene to identify samples of illegal drugs
  • Or match paint chips from vehicles thought to be involved in hit and run accidents
• What are photons?

  • It is electromagnetic radiation as a stream of packets (or quanta) of energy
  • Photons have no mass or charge - they are only described by their energy, wavelength or frequency
  • The energy of a photon is proportional to its frequency
• Energy of a photon:

  E = hf
• Intensity of electromagnetic radiation is the energy transferred per unit time per unit area

  • It depends on the energy carried by the photons,
  • the number of photons transferred each second,
  • the area on which the photons are incident
• The intensity increases:

  • when light source is made more powerful => more photons are transferred each second
  • when each photon transfers more energy
  • Higher frequency = higher energy photon
  • Shorter wavelength = higher energy photon
  • when the light is incident on a smaller area => more light energy enters the area each second
• What is an Electronvolt (eV)?

  It is the energy needed to move an electron through a potential difference of 1 volt
• Work done in eV, calculated using:

  W = V x Q

  W, energy transferred in eV

  V, potential difference in Volts

  Q, electron charge
• Absorbing and emitting photons:

  • Electrons gains and loses energy as it moves within an atom
  • Electron has a combination of kinetic energy and electrostatic potential energy
  • Electron has a negative charge and the nucleus has a positive charge -> electron is attracted to the positive charge + work must be done to move electron away from nucleus so electron has less energy near nucleus
• Electron moves further from nucleus if it gains right amount of energy by absorbing a photon => electron moves to higher energy level
• When an electron drops from a higher level to a lower energy level, it loses surplus energy by emitting a photon and moves closer to the nucleus
• Why can an electron only absorb a certain amount of energy?

  Because the possible, or allowed energies for electrons in an atom are not continuous => they are fixed and only certain energy levels are allowed
• What is atom excitation?


  it occurs when an electron/s absorb the exact amount of energy to move to a higher energy level
• What is ionisation?


  It occurs when an atom gains or loses an electron and becomes a charged particle called an ion
• What is the meaning of ionisation energy?

  the minimum amount of energy needed to remove an electron from the atom completely
• What does line spectra prove to us?

  • Electrons in atoms exist in discrete energy levels
  • Atoms can only emit photons with energies equal to the difference between two energy levels
  • Since only certain photon energies are allowed, you only see the corresponding wavelengths in the line spectrum
• What can be used as evidence for the discrete energy levels in atoms?

  • Line emission spectra + absorption spectra
  • The lines appear at discrete points which show where a light photon of specific frequency and wavelength have been absorbed or emitted
  • That shows that electrons can only absorb an exact amount of energy to be excited to the next discrete energy level
• Why is the energy always negative in spectral lines?

  • For an electron to become free, energy has to be applied

  OR

  • Zero energy is the maximum amount of energy and is when the electron is at infinity
• In a fluorescent tube: Explain how mercury vapour atoms become excited?

  • Electrons passing through tube collide with mercury electrons in mercury atom
  • transferring energy to the mercury atoms
  • causing electrons in mercury atom to move to a higher energy level
• In a fluorescent tube: How do the excited mercury atoms emit UV photons?

  • excited electron de-excites to a lower energy level
  • emitting a UV photon of energy equal to the energy difference between the levels
• In a fluorescent tube: Explain how the UV photons in the tube are converted into photons in the visible part of the electromagnetic spectrum?

  • phosphor coating absorbs UV photons and this causes electrons in the phosphor coating to excite
  • excited electrons in coating de-excite to a lower energy level emitting visible light photons equal to the difference between the levels
• What is the photoelectric effect?

  The emission of photoelectrons from a material when light of a sufficient energy/frequency is shone on it.
• Photoelectric effect:

  • The emission of electrons occurs almost instantly whereas wave theory predicts that the emission would take time
  • Photoelectric effect is an experiment that proves the particulate nature of EM radiation
  • Wave theory predicts the photoelectric effect BUT is unable to explain the fact the emission occurs virtually instantly
• What is the work function of a metal in the photoelectric effect?

  • ϕ - the minimum amount of energy required to release an electron from the surface of a metal
• What is the threshold frequency?

  the minimum photon frequency required to release a photon from the surface of a metal
• Photoelectric effect in a nutshell:

  • If energy of the photon > work function - an electron will be emitted
  • If energy of the photon < work function - an electron will NOT be emitted
  • There is a 1 to 1 interaction between electrons and photons
  • i.e. if we use a lower energy photon we can never get more than 1 photon interacting with an electron and releasing it
• Conclusions from gold leaf experiment:

  • Emission of photons depends only on the frequency of the light/EM radiation not on the intensity
  • The electron emission is almost instantaneous which means EM radiation has a particle nature
  • There is a one to one interaction between photon and electrons
• Describe the Gold Leaf Experiment to demonstrate the photoelectric effect

  • Charge gold leaf electroscope with a plastic rod
  • Charge then accumulated on the surface of the zinc plate and on gold leaf
  • Then due to electromagnetic repulsion the gold leaf arises
  • electrons on zinc plate + gold leaf repel each other
• What happens if visible light is shone on the zinc plate in the gold leaf experiment?

  • If visible light shone on zinc plate - no electrons emitted from the metal as the photon energy of visible light is below the work function of zinc
  • If intensity of visible light is increased (more photons per second) still no electrons are emitted as there is a 1 to 1 interaction between photons and electrons
• What happens if UV light is used instead of visible light?

  • As UV light has a higher frequency, photoelectrons are emitted as the photon energy of UV light is greater than the work function of the metal, zinc plate - causes gold leaf to drop and discharges electroscope
  • If intensity increases => more photons per second = more electrons emitted
  • HOWEVER: if glass is inserted, in front of UV source, the UV will not drop the leaf as glass absorbs UV light