Quantum Theory

Nota:

• The 3 laws of motion

1. After this until late 19th Century mechanics became a discipline capable of describing the motion of particles in a clear and deterministic way.
   1. (Light / Opticks was a stream of particles - Atomistic)

1. He determined, that all electromagnetism was linked, and that the speed of light could be determined

1. Light had 'wavelike' characteristics
   1. Interference Phenomena
      1. "Adding in phase waves gives mutual reinforcement - Adding out of phase waves gives cancellation"

1. 1885
   1. Discovered that the spectrum of Hydrogen, could be described by a simple mathematical formula

      Nota:

      • when light is shone through hydrogen gas, and then through a prism, the frequencies that have been absorbed by the gas are absent in the spectra. These frequencies could be predicted by a certain simple formula.

1. 1897
   1. Was the first to discover Electrons
      1. In his model of the Atom - Electrons, Neutrons and Protons. His model was unsuccessful in explaining Balmer's results
2. Tried to understand Balmer's result

1. 1900
   1. James Clerk Maxwell & Ludwig Bolzmann concluded that they cold reliably predict the overall behaviour of a complex system.
      1. Lord Rayleigh used this 'statistical physics' applied to a case of the black body

         Nota:

         • A black body is a hypothetical, whereby this 'body' can totally absorb all the radiation emitted at it, and then re-emit it, when it is being recorded. In this assumption, a good approximtion could be made to a  specially prepared oven. In doing so the answer is only dependant on the temperature of the black body.
         1. The results from the Black body experiment did not match the calculations
            1. Classical physics assumed that radiation 'oozed' in and out of the 'black body'

1. Submitted that energy was emitted or absorbed from time to time in small packets of energy of a definite size and that the size of this energy packet was directly proportional to the radiation's frequency. THE BIRTH OF PLANCK's CONSTANT

   Nota:

   • This meant that high frequency radiation could only be emitted or absorbed in events involving a single quantum of significantly high energy, explaining why high frequency events seemed unusually supressed by comparison to the classical physics.

1. In classical theory - Radiation absorbed by a sheet of metal could release electrons if the intensity or energy content of the beam was sufficient to release the electron. In this model the electron release would only be related to the intensity, not the frequency
   1. his experiment showed the opposite. Below a certain frequency a beam may emit no electrons however intense the beam is. above that frequency a beam could emit electrons however low the intensity

      Nota:

      • This puzzling behaviour became instantly understandable when you consider the beam of radiation as a stream of persisting quanta. An electron would be ejected if one of these quanta had collided with it and given up all its energy. The amount of energy in the quanta was directly proportional to the frequency, related by Planck's constant. If the frequency were too low, then there would not be enough energy transmitted in the collision to enable the electrons escape. The intensity of the beam, simply determined how many quanta were in the beam, and so how many electrons were involved in collisions and were able to escape. Increasing the intensity could alter the amount of energy in a single collision.
      1. These Quanta became 'photons'
         1. CRISIS

1. How come Alpha particles were deflected by a thin gold film

   Nota:

   • It was as astonishing as if a 15" naval round had recoiled on striking a sheet of toilet paper.
   1. J.J Thompson's Plum Pudding model (1897) of Atom's could make no sense now
      1. The Positive charge of the Gold Atoms, could not be spread out as suggested, but must be concentrated at the centre of the atom in order to provie enough repulsion to deflect an alpha particle.
         1. this gave way to the Solar System theory
            1. Which led to yet more crisis - If electrons are constantly encircling an atomic nucleus they are constantly changing direction. For this to work they would have to radiate away some of their energy and gradually decline to the centre of the nucleus. Atoms would be unstable and this process of decay the spectra should no longer be as defined as the spectra that balmer had uncovered

1. Bohr took on some of the principle thinking that Plank had used. Rather than to suppose that radiation could ooze in and out on a sliding scale he assumed a system that was rather more discrete. Electrons he proclaimed could only exist at certain permitted orbits. (against the position a classical physicist may take)
   1. Using Planck's constant he proposed how these radii could be determined. The immediate consequence was that his theory provided a more fitting model of an atom THAT WAS STABLE. His mathematical formula also led to the Balmer formula discovered almost 30 years previously

      Nota:

      • once an electron was in an orbit, corresponding to the lowest permitted radius - it had nowhere else to go so no more energy could be lost. It may have got to this point by emitting energy as it dropped from higher orbits. This energy would be emitted as a single photon.

1. He found that scattered x-rays had their frequency changed by matter. with a wave model this could not be understood.

   Nota:

   • in the classical theory, it would be argued that the scattering process would be due to electrons in the atoms absorbing and re-emitting energy from the incident waves without altering the frequency
   1. According to Planck's theory a change in energy is equal to a change in frequency which fitted, and thus Compton was able to give a quantitive explanation for his findings providing the most persuasive evidence to date of the particle like character of electromagnetic radiation.
      1. These Theories could be ignored no longer