Only light that enters the fiber core within a certain range of angles or other known as critical angle or acceptance cone, can be successfully travelling through the length of the fiber.
The light that does enter the fiber core but not within the acceptance cone will normally escape or lost into fiber optics cladding.
The size of acceptance cone is a function of the refractive index difference between the fiber optics core and cladding.
where n(core) is core refractive index and n(cladding) is cladding refractive index.
However, all these factors are being taken care with due diligence during the designing and manufacturing process of fiber optics preform.
I have been in the field of fiber optics since early 1990s. I gained fiber optics skills and knowledge via my working experience as end-user, main contractor and sub-contractor and finally as an optical fiber enterpreneur.