How is information transmitted by radio waves and through optical fibres, and what limits each?
Wireless and optical transmission: radio-wave transmission and the aerial, attenuation and noise, optical-fibre transmission, total internal reflection, and multimode versus monomode fibre.
A focused answer to WJEC A-Level Electronics wireless and optical transmission, covering radio-wave transmission and the aerial, attenuation and noise on a channel, optical-fibre transmission by total internal reflection, and the difference between multimode and monomode fibre.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this dot point is asking
Having modulated a signal, you must transmit it, and WJEC covers the two dominant methods: radio waves through the air and light through optical fibre. The specification expects you to describe radio transmission and the aerial, explain attenuation and noise, describe optical-fibre transmission by total internal reflection, and compare multimode with monomode fibre. The total-internal-reflection explanation and the multimode-versus-monomode comparison are reliable, high-mark exam content.
The answer
Radio-wave transmission
Attenuation and noise
Optical-fibre transmission
Multimode versus monomode
Examples in context
- Example 1. A transcontinental internet backbone
- Long-haul data travels on monomode fibre, where the single light path keeps pulses sharp over hundreds of kilometres between repeaters. The very low attenuation and absence of modal dispersion are why fibre, not copper, carries almost all long-distance internet traffic.
- Example 2. A short office data link
- Within a building, multimode fibre is often used because the distances are short, so modal dispersion is not a problem, and multimode equipment is cheaper. This shows the choice of fibre depends on distance and data rate, not on one type being universally better.
- Example 3. Immunity to interference in a factory
- Optical fibre carries no electrical current, so it is immune to the electromagnetic interference from heavy machinery that would corrupt a copper cable. This immunity, plus its security against tapping, is a key reason fibre is chosen in electrically noisy environments.
Try this
Q1. State the condition for light to undergo total internal reflection at the core-cladding boundary of an optical fibre. [2 marks]
- Cue. The core must have a higher refractive index than the cladding, and the light must strike the boundary at an angle greater than the critical angle.
Q2. Give one reason monomode fibre is preferred over multimode fibre for a long-distance link. [1 mark]
- Cue. Its narrow core gives essentially one light path, so modal dispersion is almost eliminated, allowing a higher data rate over long distances.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC Eduqas 20215 marksExplain how light is guided along an optical fibre, and compare multimode and monomode fibre for a long-distance high-data-rate link.Show worked answer →
Light is guided along an optical fibre by total internal reflection. The core has a higher refractive index than the surrounding cladding, so light striking the core-cladding boundary at an angle greater than the critical angle is totally internally reflected and stays in the core, bouncing along the fibre.
Multimode fibre has a wide core, so light can take many paths (modes) of different lengths. The shortest and longest paths arrive at slightly different times, spreading out a pulse (modal dispersion), which limits the data rate over distance.
Monomode fibre has a very narrow core, so light travels essentially one path, almost eliminating modal dispersion. This gives a much higher data rate over long distances, which is why monomode is used for long-haul, high-data-rate links.
Markers reward total internal reflection with the higher-index core, modal dispersion in multimode fibre, and monomode being better for long-distance high data rate.
WJEC Eduqas 20194 marksState two advantages of optical-fibre transmission over copper-cable transmission, and explain what is meant by attenuation.Show worked answer →
Advantages of optical fibre over copper: it has a much higher bandwidth (so a far higher data rate), it suffers much lower attenuation over distance (so signals travel further between repeaters), it is immune to electromagnetic interference, and it is lighter, thinner and more secure (harder to tap). Any two of these are acceptable.
Attenuation is the gradual loss of signal power as it travels along the channel, caused by absorption and scattering. It is usually measured in decibels and means the received signal is weaker than the transmitted one, eventually needing amplification or regeneration.
Markers reward two valid optical-fibre advantages and a correct definition of attenuation as loss of signal power with distance.
Related dot points
- Communication principles and modulation: the structure of a communication system, the need for a carrier, amplitude and frequency modulation, bandwidth, data rate, and noise and distortion.
A focused answer to WJEC A-Level Electronics communication principles and modulation, covering the structure of a communication system, the need for a carrier, amplitude and frequency modulation, bandwidth and data rate, and the effects of noise and distortion.
- Passive filters: the RC low-pass and high-pass filter, the cut-off (break) frequency, the frequency response and the half-power point, gain in decibels, and the Bode plot.
A focused answer to WJEC A-Level Electronics passive filters, covering the RC low-pass and high-pass filter, the cut-off (break) frequency, the frequency response and the minus 3 dB half-power point, gain expressed in decibels, and the Bode plot.
- AC signals and reactance: peak, peak-to-peak and RMS values, frequency and period, capacitive and inductive reactance, and the phase relationship between voltage and current.
A focused answer to WJEC A-Level Electronics AC signals and reactance, covering peak, peak-to-peak and RMS values, frequency and period, capacitive and inductive reactance, how reactance varies with frequency, and the phase between voltage and current.
- Instrumentation systems: sensors and transducers, the Wheatstone bridge, signal conditioning and amplification, calibration, and the use of the instrumentation amplifier.
A focused answer to WJEC A-Level Electronics instrumentation systems, covering sensors and transducers, the Wheatstone bridge for resistive sensors, signal conditioning and amplification, calibration, and the role of the instrumentation amplifier.
- Analogue-to-digital conversion: sampling, the sampling rate and the Nyquist criterion, quantisation, resolution and the number of bits, and quantisation error.
A focused answer to WJEC A-Level Electronics analogue-to-digital conversion, covering sampling and the sampling rate, the Nyquist criterion, quantisation into levels, resolution and the number of bits, and quantisation error.
Sources & how we know this
- WJEC Eduqas GCE A-level Electronics specification — WJEC Eduqas (2017)