Speed of Signals in a Wire vs Fiber Optic Cable How much faster is the transmission of a signal in a fiber optic cable than in a copper wire? I would assume fiber optic cables transmit signals at the speed of light (this begs the question, are fiber optic cables vacuum sealed or is the light moving through air, but that wouldn't make much of a difference) but how does that compare with the speed of a signal moving through a conductor? I've seen online that signals travel at about the speed of light as well. If that is the case though, why are fiber optic cables preferable?
 A: Let's state some basic facts.


*

*Whenever speed of light is given as $186,000~\rm
mi/s$ or $300,000~\rm km/s$, that's its speed in a vacuum.

*In the real world we use copper  cable or fibre 
optic  (glass) to conduct electricity or light.

*Copper slows electrical current to about $70\%$ of the
speed of light and glass slows light by about the same.
So why bother with fibre at all?
There is a problem with copper: whilst it is not magnetic
on its own, the electric current it carries creates a magnetic 
field which causes interference in nearby cables and prevents
the transmission of higher frequencies used for television and data
signals. The longer the cable and the higher the frequency
the worse it gets until  it becomes unusable.
Fiber, which is carrying a laser signal, has no magnetic
fields created and is immune from any external magnetic
fields and has a virtually unlimited capacity (or bandwidth).
Hope that goes some way to explain things.
A: I test fibre and copper cables.
The speed of a "signal" or a pulse of electricity ,or light ,through either is about
200,000 km/hr.
the problem is copper carrying current creates a magnetic field. Combined with the 
inductive effects and capacitance of cable conductors the higher the frequency the less a cable is effective.  Co ax,twisted pairs,shielded etc all work to an extent but only over shorter distance.
Fibre has no limitations compared to copper.
I was keeping maths out of my answer because its not needed to understand why we use
either cable. Copper is simple for electrical applications and lower frequency or short
distances.  Fibre for high capacity( tens if thousands of simultaneous phone calls on one fibre)  and long distances.
Thankfully our test instruments do the maths for us if we set the cable type ,or refractive index for fibre,or velocity of propagation for copper before testing.
