3.2.2 Electromagnetic radiation and quantum phenomena

3.2.2 Electromagnetic radiation and quantum phenomena

1. The photoelectric effect

   • emission of electrons from a metal surface when the surface is illuminated by light of frequency greater than the minimum value known as the threshold frequency
     • threshold frequency = f min
   • work function, Φ
     • electron can leave the metal surface if the energy gained from a single photon exceeds the work function
   • photoelectric emission occurs without delay
   • Ek (max) is the max Ek of the photo electrons
   • vacuum photocell
   • stopping potential
     • min. potential needed to stop photoelectric emission

2. Collisions of electrons with atoms

   • ionisation examples
     • alpha, beta, and gamma radiations passing through substances and colliding with the atoms in the substance
     • electrons passing through a fluorescent tube create ions when they collide with the atoms of the gas or vapour in the tube
   • 1 eV = 1.6 x 10^(-19) J
     • electron volt is a unit of energy equal to the work done when an electron is moved through a pd of 1V
   • excitation by collision
     • gas atoms can absorb energy from colliding electrons without being ionised
       • happens at certain energies

3. Energy levels and photon emission

   • Fluorescent tube
   • ground state
     • if at energy level higher than ground state, it is excited
   • de-excitation
     • photon emitted by electron when it is de-excited
       • energy of photon is equal to the energy lost by the electron and therefore by the atom
   • excitation using photons
     • electron in an atom can absorb a photon and move to an outer shell where a vacancy exists, but only if the energy of the photon is exactly equal to the gain in the electron's energy
   • line sprectra
     • each line in a line spectrum is due to light of a certain colour and therefore a certain wavelength
     • photons that produce each line all have the same energy

4. Wave-particle duality

   • wave-like nature
     • diffraction
       • light passing through a narrow slit
   • particle-like nature
     • photoelectric effect
       • when light is directed at a metal surface and an electron at the surface absorbs a photon, the Ek of the electron is increased from a negligible value to hf
   • de Broglie wavelength
     • λ = h / p = h / mv
     • evidence

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