ABSTRACT
The R. P. Feynman path integrals provide an alternative version of quantum mechanics. This approach is outlined since Feynman applied it to describe macroscopic beam divergence. Feynman was fascinated by the principle of least action and he gave a lecture on this topic in the Feynman Lectures of Physics. This principle can be used to describe the possible paths of a particle from an initial to a final point. A Feynman diagram in its simplest form depicts a sequence of events, from left to right, as a function of time. Other diagrams are 2D with space on the horizontal axis and time on the vertical axis. The diagram that is going to consider depicts the absorption and emission of light as a function of time. An additional example involves the pictorial description, using the Feynman diagrams, of the third harmonic generation given by P. N. Butcher and D. Cotter.
