Light is not only used for seeing but can also be used to send information along optical fibres. More information can be carried via an optical fibre than by sending electrical signals through metal cables of the same diameter. There is also less weakening of the signal in optical fibres, compared to electrical signals.
Light is kept in the fibre optic because every internal incident ray angle is greater than the critical angle so total internal reflection occurs. Fibre optics are made out of thin continuous strands of glass covered in an opaque sheath to stop an external light in. For glass the critical angle is 42°
