# Does the index of refraction of an optical fibre increase with the light path length?

Assuming that the fibre is flawless, does the index of refraction of an optical fiber cumulatively increase with increasing length of travel of light within the fiber? Some descriptions of refraction lead to a strong inference that this must be true but at the same time the values are usually given as material dependent and not related to the details or length of light path through the material. This question is not intended to be about the fibre per se but is about trying to understand the way that refraction actually arises in the interaction between material and light.

I am asking this question because I am not able yet to leave comments asking for clarification.

• Hi, welcome to Physics.SE! If this question is motivated by another post, it might be useful to add a link to the other post. (But I think this one stands on its own okay.) – rob Dec 15 '17 at 3:12

## 1 Answer

Index of refraction is an intensive property of a material. In a homogeneous material it doesn't depend on the amount of material or the shape you arrange the material into.

Note that the glass in optical fiber is not homogeneous. The composition varies radially, with higher dopant cocentration (and thus higher index of refraction) near the centerline of the fiber, and lower index away from the centerline. The variation might be step-like (a uniform core surrounded by a uniform lower-index cladding) as in most single-mode fibers; or it might be continuous (ideally, a parabolic shape) as in high-performance multi-mode fibers.

Also, index of refraction is a macroscopic property. It's the effect of many many atoms interacting with the light beam travelling through the material. If you look at the glass on a scale where individual atoms are relevant, you'd no longer see the glass as having a uniform effect on the light's propagation velocity. (This is similar to how a current in a river is the macroscopic view of vast numbers of water molecules travelling in mostly random directions which only happen be slightly biased toward travel in the direction of current flow).

One thing that might have confused you is that optical path length does depend on the index of refraction of the material the light is travelling through. Optical path length is the length of vacuum a light beam would have had to travel through to have the same phase advancement as it had travelling through some material. So if a beam travels through 1 cm of glass with index 1.5, then the optical path length is 1.5 cm. If it travels through 1 cm of glass with index 1.6, then the optical path length is 1.6 cm. This is because the optical path length depends on the index, not because the index depends on the dimensions of the piece of glass.

• I'm trying to get to why the light is slowed i.e. mass effect, electric effect, gravity effect, etc. The light through the transparent medium has a compressed but not circuitous path right? I'm trying to visualize the simplest case monochromatic single photon/wave/wave packet ignoring dispersion if possible. You confirmed my understanding that the index can't be cumulative. – DMac Dec 15 '17 at 4:20
• Classical view: As the light wave passes through, it excites the charged particles in the material (electrons) to vibrate a bit. As they vibrate, they emit an EM wave of their own, at the same frequency as the original wave. The sum of the original wave and the waves from the excited electrons is a wave that's a bit delayed compared the original wave. – The Photon Dec 15 '17 at 5:12