點解用光纖傳訊號會快過銅線？

The speed of signals through a copper cable (or transmission line) is NOT the same as the speed of light. Generally, the signal speed is affected by the capacitance of the cable, which in turn depends on the material making up the dielectric surrounding the copper wire (i.e. the insulation). Signal speed in copper cables varies from 40% to 70% of the speed of light.

The speed of light signal in optical fibre is just the reciprocal of the refractive index of the fibre core multiply by the speed of light. For glass with refractive index 1.5, the signal speed is thus 67% of the speed of light.

Thus the speeds of signal travelling through copper transmission lines and optical fibres are just comparable (you could say that in general, signals travel a bit faster in optical fibres than in copper lines). But be aware that it is not the speed, it is the bandwidth (頻寬) that makes the difference. Optical fibre has a potential bandwidth thousands of times wider than with copper conductors. For example, coaxial copper cables can have bandwidth of order of GHz, but optical fibres can go up to 10 GHz easily. Thus optical fibres can accommodate more channels and also enables the transmission of video signals that requires much wider bandwidths.

when a electrical circuit is completed ,e-field will be developed and produce a signal to make electrons moving around the circuit

That whole e-field produced the signal speed is only about 10^8ms-1,which is slower than light speed a lot (3x10^8ms-1).

so why using the optical fiber transmit signal is faster than using copper wire.

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