A fibre optic cable is used to transmit signals. When a short pulse of light passes along a fibre, it spreads out, which limits the rate of transmission of signals down the fibre.
a) Suggest two reasons why the pulse of light might spread out. [2]
b) A fibre of length 10.0 km is illuminated with red light from an led which is turned on and off repeatedly for equal amounts of time. The speed of the pulse of light ranges from 1.95 × 10⁸ m/s to 2.05 × 10⁸ m/s. Calculate the range of times taken for the pulse to travel down the fibre optic. [1]
c) What is the maximum frequency of the led so that the pulses arrive without overlapping? [3]
d) The wavelength the LED emits is 1310 nm in air. Calculate the frequency of the light used. (c = 3.0 × 10⁸ m/s) [1]
e) The frequency of light at the red end of the spectrum is 4 × 10¹⁴ Hz. Explain in what part of the spectrum the 1310 nm of part (d) is to be found. [2]