Electromagnetic Radiation and Quantum Phenomena

Electromagnetic Radiation and Quantum Phenomena

1. The Photoelectric Effect

   • The photoelectric effect is when a free electron on the surface of a metal absorbs enough energy to break the bonds holding the electron there and the electron is released. The energy comes from a high frequency of light
     • The Threshold frequency is the minimum frequency need to allow an electron to leave the surface of a metal.
   • The photoelectric effect caused a few problems because it couldn't be explained using the wave theory of light
     • According the wave theory, the free electrons should gain a little bit of energy with each incoming wave, so the electrons would've gradually have enough energy to leave the surface. This does not explain the threshold frequency
       • The photon theory can explain the threshold frequency. When the energy gained from a photon is greater than the work function, an electron is emitted. When the photon does not carry enough energy, the metal simply heats up
     • Wave theory also states that light intensity is proportional to energy, thus a higher intensity would mean more energy being transferred to the electrons. This does not explain how kinetic energy- which should increase with intensity- is only dependant on frequency
       • the photon theory also explains how the kinetic is independent of the intensity. the electrons only absorb one photon at a time, an increase in intensity only increases the amount of photons per second in a area, not the energy supplied
     • It could, however, be explained using the Photon model of light, as it describes the EM waves existing in discrete packets of energy, having a one on one interaction with electrons, transferring all of their energy into one electron
   • The work function is the minimum amount of energy needed for an electron to leave the surface of a metal
   • Ek(max) is the maximum kinetic energy of the photoelectrons.

2. Collisions of Electrons with atoms

   • Excitation is when an electron gains exactly enough energy to move up between two energy level in an atom. Ionisation is when the electron gains enough energy to completely leave an electron
     • The ionisation energy is the amount of energy required for an electron to leave the atom from the ground state
   • Fluorescent tubes contain mercury vapour which has a high voltage applied. This voltage accelerates free electrons which ionises mercury vapour, producing more free electrons
     • When these free electrons collide with other electrons in the mercury atoms, they excite them to another energy level. When these electrons return back to their original energy level they release a photon in the UV range
       • When these photons interact with the electrons in the phosphor atoms coating, they excite the electrons to an higher energy level. When the the electrons return to the original energy level they release an photon in the form of visible light
   • An electron volt is defined as the kinetic energy carried by an electron after it has accelerated through a potential difference of 1 volt

3. Energy levels and photon emissions

   • When you use a prism or a diffraction grating to split the light produced by the fluorescent tube, you get a light spectrum. The line agaisnt the black background correspond to a wavelength emitted by the light
   • Evidence to show the existence of energy levels can be found in light spectra. At cool temperatures, most electrons will be at their ground state. They can move up when they absorb photons of equal energy to the difference between two energy levels.
     • When electrons absorb photons of the same wavelength, they become excited to a higher energy level and the wavelength becomes missing from the emission spectra
       • If you compare the emission and absorption spectra of the same gas, the bright lines on the emission spectra and the black lines of a absorption spectra line up

4. Wave Particle Duality

   • Wave particle duality suggests that "wave like" light can show particle behaviours and typical particles can show wave like properties. This is shown in two instances where the observation cannot be explained using their typical models.
     • Electron diffraction is observed when diffraction patterns occur when accelerated electrons interact with a graphite crystal. This cannot be explained with the particle theory of light as the patterns occur through wave interference which particles can not do
       • The diffraction lines become more spaced out when the wavelength is wider, which is affected by the speed
     • Photoelectric effect, as stated above-east, only works if light acts like a particle like photon
   • The de Broglie wavelength is an equation which finds the wavelength of a particle based on particle features (mass and velocity). The de Broglie wavelength can be used to find out the size of an object needed for a particle to interact and diffract
     • de Broglie hypothesis was only accepted after it was peer reviewed and was tested many times. It is accepted now until conflicting evidence is given

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