(a(i))[3]
In an optic fibre, the signal-to-noise ratio must stay at or above $24\,\text{dB}$. The mean noise power in the fibre is $5.6\times10^{-19}\,\text{W}$. Calculate the minimum effective signal power in the optic fibre.
(a(ii))[3]
The fibre loses $1.9\,\text{dB km}^{-1}$ by attenuation. Calculate the greatest uninterrupted fibre length for an input signal of power $3.5\,\text{mW}$.
(b)[1]
Suggest why infra-red radiation, rather than ultraviolet radiation, is used for long-distance communication through optic fibres.