A-LevelPhysicsWaves9702/42 · Feb/Mar 2020 · Q57 Marks

5 (a) State two advantages of the transmission of data in digital form, rather than analogue form. (b) Optic fibres are used for the transmission of data.

(a)State two advantages of the transmission of data in digital form, rather than analogue form. 1. 2. [2][2]
(b)(i)A signal in an optic fibre is carried by an electromagnetic wave of frequency 1.36 × 10¹⁴ Hz. The speed of the wave in the fibre is 2.07 × 10⁸ms⁻¹. For this electromagnetic wave, determine the ratio: wavelength in free space wavelength in fibre ratio = ........................................................................ [2][2]
(b)(ii)The attenuation per unit length of the signal in the fibre is 0.40dBkm⁻¹. The input power is 1.5mW and the output power is 0.060 mW. Calculate the length of the fibre. length = ................................................................ km [3][3]

Mark scheme

Marking points from the official mark scheme (Feb/Mar 2020). The mark or mark type is shown in brackets.

[1] noise can be filtered out / noise can be removed / signal can be regenerated
[1] can carry more information per unit time / greater rate of transmission of data
[1] can have extra bits of data to check for errors
[1] can be encrypted
[1] v ∝ λ OR ratio = V_air / V_fibre
[1] = 3.00 × 10⁸ / 2.07 × 10⁸ = 1.45
[1] attenuation = 10 log (P_input / P_output)
[1] 0.40 × L = 10 log (1.5/0.06)
[1] L = 35 km

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