Optical path length, definition

It is clear from these definitions that over arbitrary distance z along the waveguide, the accumulated path length, optical path length and number of... 

A definite absorption path length L can be realized with a retroreflector, placed at a distance L/2 from the source. Another possibility is to place the laser in an airplane (or satellite) and the receiver on the ground. Figure 14.4 illustrates a possible experimental setup. The laser output is expanded by a telescope in order to decrease the divergence of the beam. The retro-reflector reflects the beam exactly back into a beam splitter and into a polychromator. With a diode array at the spectrometer output (optical multichannel analyzer see Sect.4.5.9) the whole spectral range of the multiline laser can be recorded simultaneously. This diode array also allows measure-... 

Following Atkins [68], the propagation of particles follows a path dictated by Newton s laws, equivalent to Hamilton s principle, that particles select paths between two points such that the action associated with the path is a minimum. Therefore, Fermat s principle for light propagation is Hamilton s principle for particles. The formal definition of action is an integral identical in structure with the phase length in physical optics. Therefore, particles are associated with wave motion, the wave-particle dualism. Hamilton s principle of least... 

---