Know that many important systems of communications rely on electromagnetic radiation including: (a) mobile phones (cell phones) and wireless internet use microwaves because microwaves can penetrate some walls and only require a short aerial for trans

3.3 Electromagnetic Spectrum

All electromagnetic (EM) waves travel in vacuum at the same speed

\(c = 3.0 \times 10^{8}\ \text{m s}^{-1}= \lambda f\)

where \(c\) is the speed of light, \(\lambda\) the wavelength and \(f\) the frequency.

Summary table – syllabus layout (AO1)

Ionising vs. non‑ionising radiation (key AO2 point)

• Non‑ionising: Radio, microwave, infrared, visible – photons do not have enough energy to remove tightly bound electrons.
• Ionising: Ultraviolet (short‑wave), X‑ray, gamma – photon energy ≥ 10 eV; capable of ionising atoms and damaging biological molecules.

Why particular regions are chosen for communication (AO2)

• Microwaves (≈ 300 MHz – 30 GHz)

  • Wavelengths 1 cm – 30 cm → compact “short” aerials on phones, routers, satellites.
  • Short enough to provide large bandwidth → high data‑rate capacity.
  • Can diffract around obstacles and penetrate non‑metallic walls, giving reliable indoor coverage.
  • Typical communication uses: mobile phones, Wi‑Fi (2.4 GHz, 5 GHz), satellite TV, radar.

• Radio waves (≈ 3 kHz – 300 MHz)

  • Longer wavelengths (≈ 10 cm – 1 km) pass easily through walls and other non‑conductive materials.
  • Low‑power operation → ideal for battery‑driven short‑range devices (Bluetooth, RFID).
  • Bandwidth is smaller than microwaves but sufficient for voice, simple data or control signals.
  • Typical communication uses: FM/AM broadcast, GPS, Bluetooth (2.4 GHz ISM band), RFID.

• Visible / Near‑infrared light in optical fibres

  • Silica glass is almost transparent between 0.75 µm and 1.6 µm; attenuation only a few dB km⁻¹.
  • Light is confined by total internal reflection → loss‑free transmission over tens of kilometres.
  • Very high carrier frequency (≈ 200–400 THz) gives theoretical bandwidth of several THz → gigabit‑per‑second links.
  • Typical communication uses: broadband internet, cable TV, inter‑city data links.

Digital vs. analogue signals (AO2)

• Analogue signal: continuous variation of amplitude, frequency or phase that directly represents the information.
• Digital signal: series of discrete values (normally binary 0 / 1) that encode the information.

Exam‑relevant advantages of digital signalling (AO2 – bullet points used in past papers)

• Higher data‑rate capability – many bits can be packed into a short time‑interval.
• Noise resistance – errors can be detected and corrected with error‑checking codes.
• Easy regeneration – the original signal can be reproduced without loss of quality (regenerators in telephone networks).
• Flexibility – the same physical carrier can carry voice, video or computer data.

Signal, carrier and basic modulation (AO2)

• Signal: the information to be transmitted (voice, music, video, computer data).
• Carrier wave: a high‑frequency EM wave that, by itself, carries no information.
• Modulation: varying a property of the carrier in step with the signal so that the information can be sent over the chosen part of the spectrum.

Simple modulation types examined at IGCSE level

• Amplitude Modulation (AM) – carrier amplitude is varied.

  
  Mathematically: \(s{\text{AM}}(t)=Ac[1+m\,s(t)]\cos(2\pi f_c t)\)

  where \(Ac\) is carrier amplitude, \(fc\) carrier frequency, \(s(t)\) the base‑band signal and \(m\) the modulation index.

• Frequency Modulation (FM) – carrier frequency is varied.

  
  Instantaneous frequency: \(fi(t)=fc + k_f s(t)\)

  
  Resulting wave: \(s{\text{FM}}(t)=Ac\cos\!\big(2\pi fc t + 2\pi kf\!\int_0^{t}\!s(\tau)\,d\tau\big)\)

  where \(k_f\) is the frequency‑deviation constant.

Comparison of common communication systems (AO2)

Links to other parts of the Cambridge IGCSE 0625 syllabus

• Section 4.5 Electromagnetic effects – induction in transformers and generators uses high‑frequency EM waves; heating in microwave ovens relates to the dielectric loss discussed in 4.5.4.
• Section 5.2 Radiation safety – the health‑risk table above is directly relevant when answering questions on ionising vs. non‑ionising radiation and safety limits.
• Section 6.1 Waves – the relationship \(c = \lambda f\) and the concepts of frequency, wavelength and speed are revisited throughout the course, especially in the context of wave propagation and refraction.