| Date | May 2017 | Marks available | 2 | Reference code | 17M.3.SL.TZ1.8 |
| Level | Standard level | Paper | Paper 3 | Time zone | Time zone 1 |
| Command term | Calculate | Question number | 8 | Adapted from | N/A |
Question
Communication signals are transmitted through optic fibres using infrared radiation.
State two advantages of optic fibres over coaxial cables for these transmissions.
Suggest why infrared radiation rather than visible light is used in these transmissions.
A signal with an input power of 15 mW is transmitted along an optic fibre which has an attenuation per unit length of 0.30 dB\(\,\)km–1. The power at the receiver is 2.4 mW.
Calculate the length of the fibre.
State and explain why it is an advantage for the core of an optic fibre to be extremely thin.
Markscheme
longer distance without amplification
signal cannot easily be interfered with
less noise
no cross talk
higher data transfer rate
[2 marks]
infrared radiation suffers lower attenuation
[1 mark]
loss = \(10\log \frac{{2.4}}{{15}}\) «= −7.959 dB»
length = «\(\frac{{7.959}}{{0.30}} = \)» 26.53 ≈ 27 «km»
[2 marks]
a thin core means that rays follow essentially the same path / OWTTE
and so waveguide (modal) dispersion is minimal / OWTTE
[2 marks]
