What does optical path length represent?

Optical path length is a concept that quantifies the effect of the medium on the propagation of light. It is defined as the product of the physical distance that light travels through a medium and the refractive index of that medium.

When light travels through a medium, its speed is reduced compared to its speed in a vacuum, and the refractive index is a dimensionless value that expresses this reduction in speed. Multiplying the physical distance by the refractive index effectively represents the "optical distance" that light travels in that medium. This is important in various applications, such as calculating the phase shift in interferometry or understanding light propagation in complex media.

Thus, the correct reasoning confirms that optical path length accounts for both the travel distance and the medium's properties through which the light is propagating.