Light in glass: signals without electrons

A fibre optic cable is a thin strand of glass. Instead of pushing electrons, you send pulses of light down it. The light stays inside because of one trick: at a shallow enough angle, light hitting the boundary from glass into air cannot get out and is reflected back.

The rule is about angles, measured from the line perpendicular to the surface. Meeting the boundary at a shallow angle, light bends and escapes. Meeting it at a steep angle, it cannot escape at all and reflects back inside. That is total internal reflection, and it is the whole basis of fibre optics.

# Two facts about light meeting a surface: # # leaving glass into air # - shallow angle -> light bends and escapes # - steep angle -> light cannot escape, it reflects back (total internal reflection) # # the angle is measured from the line perpendicular to the surface # A fibre keeps the light shallow enough every time it hits the wall, # so every bounce reflects instead of leaking out.

A fibre has two layers: a core and a cladding around it, made of glass with slightly different properties. The light is launched into the core at a shallow angle, so every time it reaches the cladding wall it reflects back instead of escaping. Kilometres of glass, and the light never leaves the core.