Quantum Phenomena

Quantum Phenomena

1. The photoelectric effect

   • Heinrich Hertz discovered the effect when producing a detecting radiowaves
     • He found the spark in the spark gap detector was stronger with ultraviolet radiation directed at the gap
       • Further research showed that electrons were emitted from a metal when em radiation at a certain frequency was directed at it
   • Photoelectric emission does not take place if the frequency of the radiation is below the threshold frequency
   • The number of electrons emmitted is proportional to the intensity of the radiation, as long as it is above the threshold frequency
   • Photoelectric emission occurs without delay as soon as the incident radiation is directed at the surface
   • Einstein's photon theory explained the effect. He said an electron is emitted if the energy gained by an electron from a photon exceeds the work function

2. Collisions of electrons with atoms

   • Adding or removing electrons from an atom creates an ion
     • An ion is a charged atom, the number of electrons in an ion is not equal to the number of protons
     • Alpha, beta and gamma all create ions when they pass through a substance and collide with it's atoms
     • Electrons in a fluorescent tube create ions when they pass through the gas or vapour in the tube
   • An electron volt (eV) is a unit of energy equal to the work done when an electron passes through a pd of 1V
   • Excitation is the process of gas atoms gaining energy from colliding electrons without being ionised

3. Energy levels in atoms

   • Lowest energy state is called the ground state
     • When a ground state atom absorbs energy, an electron moves to a higher energy state (excited state)
     • An excited atom will eventually de-excite to fill the vacancy in the shell the excited electron moved from
       • It emits a energy in the form of a photon and therefore moves to a lower energy level (de-excitation)
   • An atom can excite by absorbing a photon, as long as the energy gained is exactly equal to the difference between the two energy levels
   • A fluorescent tube is a glass tube filled with mercury vapour at low pressure. it emits visible light because the mercury atoms emit visible light as they de-excite which excite the fluorescent coating

4. Energy levels and spectra

   • Shining a beam of white light into a glass prism create a full, continuous spectrum of colours
   • Using a tube of glowing gas as a light source creates discrete lines instead of a continuous spectrum
     • The wavelength of the lines is determined by the atoms of the element
       • By measuring the wavelengths of the light, we can identify the element because of it's unique energy levels
   • Energy of emitted photon hf = E1 - E2
   • Helium was discovered by measuring the wavelengths of line spectra produced from sunlight

5. Wave-particle duality

   • Light consists of photons, particle - like packets of em waves. Therefore it can behave as either a particle or wave
   • The wave - like nature can be observed in diffraction when light passes through narrow slit
   • The particle - like nature can be observed in the photoelectric effect
   • de Broglie proposed that matter particles could also have a wave-like nature
     • The wave-like nature of electrons was discovered when they were diffracted through a narrow slit