Optical path length = index * thickness. If you are keeping the 60mm fixed after introducing the 5mm N-BK7 window, you will see an increased optical path length. For 5 mm of air, OPL = 5 mm. Now replace air with N-BK7 and the same space will have an OPL = (1.517)(5 mm) = 7.585 mm, which is an increase of 2.585 mm. 

Hi Sean, thank you for you reply. I’m still a little confused. Does this mean the beam of light physically travels an additional 2.585 mm simply by passing through a 5mm thick material of higher density (refractive index)? It was my understanding its speed would be reduced while travelling through the higher index material, but would still travers the same distance. Essentially the time of flight would increase, but the the path length would stay the same. Forgive me if i’m not undertesting some fundamental physics concept!

The result is a change in phase at the detector, not physical distance traveled. The light wave travels through more periods in the glass than it would in the same thickness of air. Does that help? 

@Sean Turner is correct. Optical path length is defined as the product of the index of the material and the physical distance the light travels.

@pricecj, I think you’re getting caught up on the definition. The concept of optical path length has its roots in Fermat’s principle. I recommend the RP-Photonics article. 

Note that the implication is that light acquires the same change in optical phase as it would in free space. In other words, for an index of 1.5, light has to travel a factor of 1.5X further in air to experience the same change of phase.

Hi @Sean Turner and @Michael.Young .Many thanks for your clarification, this has helped greatly! Going back to my example; when comparing the two systems, for the light that passes through the material to have the same phase at the detector as the light in the system that passes through no additional material, the detector would have to move 2.585 mm closer to the source (in this instance)?

FYI, this setup is a precursor to modelling a very simple interferometer, where I would like to match the path lengths of the measurement and reference arms so they are both in-phase when the recombine at a detector. I just wanted to make sure that the optical path length value returned by the NSRA merit function is number I need to be concerned with, not the physical path length value.

Many thanks for your time.