Quantum Phenomena

Quantum Phenomena

1. Wave or Particle

   • A Photon is a Quantum of EM radiation
     • E = hf = (hc)/wavelength
       • Energy carried by Photons = Planck's Constant x frequency (Hz)
       • h = 6.63x10^-34 Js
       • c = speed of light in a vacuum = 3.00x10^8 m/s
     • eV=(1/2)mv^2
       • An electronvolt is defined as: the kinetic energy gained by an electron when it is accelerated through a potential of 1V
         • 1eV = 1.6x10^-19J
   • The Threshold Voltage of an LED is the voltage needed to give the electrons the same energy as a photon emitted by the LED. All of the electron's KE after it is accelerated over this potential difference is transferred into a photon

2. The Photoelectric Effect

   • Shining Light on a Metal Surface can release Electrons
     • Free electrons on the surface of the metal absorb energy from light, making them vibrate - If an electron absorbs enough energy , the bonds holing it to the metal breaks and it's released - The electrons emitted are called Photoelectrons
       • If no photoelectrons are released the radiation is below the threshold frequency
       • The photoelectron's KE ranges from 0 - a maximum value
         • This max value depends on the frequency of the radiation
       • The number of photoelectrons emitted is proportional to the intensity of the beam
   • It can't be explained by wave theory (energy transferred by the light would be evenly spread over the surface so all electrons would leave at once)
     • But it can be explained by Photon Theory (if one of the photons collides with a free electron the electron will gain energy equal to hf
       • Work Energy Function - Energy needed for an electron to break the bonds holding it to the metal
   • f = work function/h
     • f = threshold frequency if it's below this the metal just heats up
     • hf = work function + (1/2)mv(max)^2
       • Independent of Intensity

3. Energy Levels and Photon Emission

   • Electrons in atoms only exist in energy levels (n=1 being the ground state) - Electrons move down energy levels be emitting a photon - because there's only certain energy levels the photons only have certain wavelengths - The energy carried by each photon is equal to the difference between the 2 energy levels
   • If you heat a gas to high temperatures, many electrons move to higher energy levels - when they return they emit protons - if you split the light you get an emission spectra (spectrum of the wavelengths of the photons it emits)
   • Shining White light through a cool gas gives an absorption spectrum
     • This matches up with the emission spectra of the gas

4. Wave - Particle Duality

   • Interference and diffraction show light as a wave
     • The Photoelectric effect shows light as a Particle
       • De Broglie came up with the wave-particle duality theory: If 'wave-like' light shows particle properties, 'particles' should be expected to show wave-like properties
         • wavelength = h/mv
         • The De Broglie wave of a particle can be interpreted a 'probability wave'
   • Electron Diffraction shows the wave nature of electrons
     • Particles don't show wave-like properties all the time
       • You only get diffraction if it interacts with something the same size as it's DBwavelength

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