Aim of quantum theory

A major aim of quantum theory is the conversion of an abstract quantity into something measurable by experiment.

For the purposes of this website a quantity M is defined - this is known as the matrix element - where M2 is proportional to the transfer rate (or speed) of the optical process.

Feynman diagram

Named after Richard Feynman, one of the founders of quantum electrodynamics. These diagrams are a pictorial representation of the light-molecule interactions.

Feynman diagram for Rayleigh scattering

Here (on the left), the vertical line repesents the molecule and the curvy lines the light; the light-molecule interactions are denoted by the black dots and time, t, proceeds up the figure.

Starting from the bottom:

  • Region I (before interaction i) - the molecule is unexcited (blue) and a single photon of light is incoming
  • Region II (between interactions i and j) - the light is absorbed by the molecule and it moves into an excited state (red)
  • Region III (after j) - light is re-emitted by the molecule and it returns to the unexcited state (blue)
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