Physics - Quantum Phenomena

• What do waves transfer?

  Energy
• What is threshold frequency?

  The minimum frequency of photon required to eject electrons from a metal surface.
• What is work function?

  The minimum energy of photon required to emit electrons from the surface of a metal.
• What is stopping potential?

  The minimum negative voltage applied to the anode to stop the photocurrent. The maximum kinetic energy of the electrons equals the stopping voltage, when measured in electron volt
• Waves do not transfer matter, they transfer energy.
• Electromagnetic waves are oscillating electrical or magnetic field. There are no particles needed so these waves travel through a vacuum.
• Mechanical waves are all waves made up of vibrating particles.
• Particles vibrate around a fixed position in a wave.
• In a transverse wave, particle oscillation is perpendicular to the direction of the wave.
• In a longitudinal wave, particle oscillation is parallel to the direction of the wave.
• What is the amplitude of a wave?

  The maximum magnitude of displacement. It is the distance from the rest position to the peak or trough.
• What is the period of a wave?


  It is the time taken for a point on one wave to get to the same point on the next wave. It is measured in seconds.
• What is wavelength?


  Length of one oscillation. The distance between one point on one wave to the same point on the adjacent wave. It is measured in metres.
• What is frequency?

  Number of waves per second that pass a set point OR the number of oscillations from a source per second. Measured in Hertz.
• Light as we know is a wave but due to wave, particle duality it can act as a particle of light called a photon. This occurs in the photoelectric effect.
• The photoelectric effect - If you shine radiation of large enough frequency onto the surface of a metal it will instantly emit photoelectrons. For most metals the necessary frequency falls in the UV range. Because of the way atoms are bonded together in metals, metals contain free electrons that are able to move about the metal. The free electrons near the surface absorb energy from the radiation making them vibrate. If they absorb enough energy, the bonds break and photoelectrons are released.
• hf = Φ + E(k max)

  hf = photon energy.

  E(k max) is the kinetic energy of particles emitted equal to 1/2mv^2 and is equal to the stopping potential.

  Φ is work function.

  If 0 KE, Φ = hf
• Photoelectric effect conclusions:

  1. For a given metal, no photoelectrons are emitted if the radiation has a frequency below a certain value
  2. Value of maximum kinetic energy increases with frequency of radiation
  3. Intensity of radiation is the amount of energy per second hitting an area of a metal. Maximum kinetic energy of the photoelectrons is unaffected by intensity of radiation.
  4. The number of photoelectrons emitted per second is proportional to the intensity of radiation.
• Stopping potential - E(k max) can be measured using stopping potential. Photoelectrons emitted can be made to lose their energy by doing work against an applied potential difference. The stopping potential is the potential difference needed to stop the fastest moving electrons travelling with kinetic energy.
• Energy Levels - Electrons in an atom exist onloy in certain defined energy levels. Each level is given a number with n=1 or n=0 representing the ground state. We say an electron is excited when it goes to an energy level greater than the ground state. Electrons move down energy levels by emitting a photon. The energy of the photon emitted can only take a certain defined value equal to the difference in the energy levels it has moved.
• On an energy level diagram, the energy on the right of n=(number) is the energy needed for the electron to leave orbit (ionise).
• Ionisation is when an electron has been removed from an atom the atom is ionised. The energy of each energy level within an atom shows the amount of energy needed to remove an electron from that level. The ionisation energy is the amount of energy needed to remove an electron from the ground state of an atom.
• Fluorescent tubes - Use the excitation of electrons and photon emission to produce visible light.

  1. A high voltage is applied across a fluorescent tube containing mercury vapour inside. This high voltage accelerates fast moving electrons.
  2. This flow of free electrons collide with the electrons in the mercury atoms. The atomic mercury electrons are excited to a higher energy level
  3. These excited electrons return to their ground state in one big step emitting high-energy photons in the UV range. The photons emitted have a range of energies and wavelengths corresponding to different transitions of electrons.
  4. A phosphorus coating on the inside of the tube absorb these photons exciting its electrons to much higher energy levels. These electrons de-excite in small stages emitting lower energy visible light photons.
• Line emission spectra - If you split light from a fluorescent tube with a prism or diffraction grating you get a line spectrum. Different gratings and prisms work by diffracting light of different wavelengths at different angles. A diffraction grating produces a better image than a prism. A line emission spectra is seen as a series of bright lines against a black background. Each line corresponds to a particular wavelength of light emitted by the source.
• Line spectra provides evidence that electrons in atoms exist in discrete energy levels. Atoms can only emit photons with energies equal to the difference between two energy levels. You see the corresponding wavelengths in the line spectrum.
• The spectrum of white light is continuous. If you split the light up with a prism, the colours merge into eachother. Hot things emit a continuous spectrum in the visible and infrared. All wavelengths are allowed because electrons aren't confined to energy levels in the object producing a continuous spectrum.
• Line absorption spectrum - You get a line absorption spectrum when light with a continuous spectrum of energy passes through a cool gas. At low temperatures, most of the electrons will be in their ground states. Photons of the correct wavelength are absorbed by the electrons to excite them to a higher energy level. These wavelengths are then missing from the continuous spectrum when it comes out the other side of the gas. A continuous spectrum has black lines corresponding to the absorbed wavelengths.
• Diffraction

  • When a beam of light passes through a narrow gap it spreads out. This is diffraction. This can only be explained using waves. If the light was acting as a particle the particle may not get straight through or would just pass straight through and the beam would be unchanged.

  The photoelectric effect

  • The results of the photoelectric effect experiments can only be explained by thinking of light as a series of particle photons. A photon has a one to one reaction with an electron giving all of its energy.
• Wave-particle duality theory - de Broglie said if 'wave like' light showed particle properties then 'particles' like electrons should be expected to show wave - like properties

  The de Broglie equation relates a wave property to a moving particle

  λ = h/mv

  His theory wasn't accepted originally and other scientists evaluated his work before he published it and it was tested through experiments like electron diffraction where it was finally accepted.
• Electrons act as a particle when they are deflected by a magnetic field and act as a wave when they are diffracted.
• Electron diffraction - Diffraction patterns can be observed using an electron diffraction tube. Electrons are accelerated to a high velocity in a vacuum and then passed through a thin graphite crystal. As they pass through the spaces of the atoms in the crystal, they diffract like waves passing through a narrow slit to produce a pattern of rings.

  The spread of the lines in the diffraction increases if the wavelength is greater (assuming the wavelength of the wave is smaller than the gap it is diffracted through). A smaller acceleration voltage gives wider rings due to less diffraction as they are slower. Increase electron speed and the diffraction circles squash together towards the middle. This fits with de Broglie's equation. In general λ for electrons accelerated in a vacuum tube is about the same size as electromagnetic waves in the x-ray part of the spectrum.
• An electron volt is the work done to accelerate an electron through a potential difference of 1V. 1eV is equal to the charge of an electron.
• key facts in notebook.
• Define an electron volt.

  The work done to accelerate an electron through a potential difference of 1V