How does light travel down a fibre-optic cable?

At the core of the cable is a strand of plastic or pure optical glass about 0.01mm in diameter. Surrounding it is a highly reflective cladding with a different refractive index to that of the core. The refractive indices of each material are engineered to ensure that light always reflects back off the cladding and is never absorbed by it, regardless of whether the cable is straight or bent round. It's a bit like carrying a torch through a long winding tunnel lined with perfect mirrors.

@askmenow