Quantum Phenomena

Quantum Phenomena

1. Photoelectricity

   • Puzzles
     • Photoelectric emmission of electrons doesnt occur if the frequency of the incident EM radiation is below the threshold frequency, which depends on the metal
     • Number of electrons emitted per second is proportional to the intensity of the incident radiation, provided the frequency is greater than the threshowld frequency.
     • Photoelectric emission occurs without delay as soon sa the radiation is directed at the surface
     • The wave theory of light
       • Cant explain
         • Threshold frequency
         • Why photoelectric emission occurs without delay
       • Incorrectly predicted
         • Emission would take place with waves of any frequency
         • Emission would take longer using low intensity waves than using high intensity waves
   • What is it?
     • Emmision of electrons when EM radiation above a certain frequency is directed at the metal
     • Discovered by Hertz
   • Einstein's explaination
     • Photon theory of light
       • Proposed by Einstein in 1905
       • Explains photoelectricity
       • Light composed of photons
         • Energy equal to hf
       • For electromagnetic waves of wavelength λ
         • E = hf = hc/λ
     • To explain photoelectricity
       • When light is incident on a metal surface, an electron at the surface absorbs a single photon from the incident light, gaining an energy equal to hf
       • An electron can leave the surface if the energy gained from a single photon exceeds the work function of the metal - the minimum energy required by an electron to secape from the metal surface
       • The maximum KE of an emitted electron must = hf - work function
   • Planck
     • Solved the unltaviolet catastrophe
       • Intensity of radiation varied with wavelength, but te theory of EM waves predicted infinite intensity at UV wavelengths
       • Energy from each vibrating atom is quantised - only certain levels of energu are allowed
       • Energy could only be in multiples of a basic amount, or quantum, hf, where f is the frequency of vibration of the atom and h is a constant
   • Conduction electrons
     • KE depends on temperature of metal
       • At 300K - 6x10^-21J
     • Work function
       • Minimum energy needed by a conduction electron to escape from the metal surface when the metal is at 0 potential
       • When an electron absorbs a photon, its KE increases by an amount equal to the energy of the photon
         • If this exceeds the work function of the metal, the conduction electron can escape the metal
         • If not, the electron collides with electrons and cations and loses its extra KE

2. Excitation

   • How to show
     • Using gas-filled tubes with a metal grid between the filament and the anode
   • Atoms can absorb energy from colliding electrons without being ionised