Return loss of transmission line and optical fiber

[yykcw]

In wikipedia, it said for optical fiber, a loss that takes place at discontinuities of refractive index, especially at an air-glass interface such as a fiber endface. At those interfaces, a fraction of the optical signal is reflected back toward the source.
But what I learned in tranmission line is when the line is terminated with an open circuit, all the wave will be reflected back.
If a section of a fiber is separated by a layer of air, isn't that considered as open circuit and all the light will be reflected back?

 

[Baluncore]

It depends on the degree of mismatch in the transmission line. An open circuit in an electrical line will reflect almost all the energy, but a minor variation will only reflect a minor proportion of the incident energy.

With optic fibre a narrow air gap does not represent an open circuit because light can cross it. But there are two mismatched impedance interfaces to cross, one on each side of the gap. The reflection coefficient will often be highly depend on the width of the gap, for example, a quarter wave length gap may cancel the two reflected waves.

http://en.wikipedia.org/wiki/Time_domain_reflectometer#Explanation