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

Learning objective

Understand that most modern communication systems use electromagnetic (EM) radiation, be able to identify which part of the spectrum each system uses, and recognise the basic properties, health considerations and digital nature of these systems as required by the Cambridge IGCSE 0625 syllabus.

1. Fundamental concepts

• Speed of EM waves: in vacuum all EM waves travel at

  c ≈ 3 × 10⁸ m s⁻¹. In a material the speed is c / n, where n is the refractive index.

• Relationship between frequency, wavelength and energy

  
  c = f λ  and  E = hf (h = 6.63 × 10⁻³⁴ J s).

  Higher frequency → shorter wavelength → higher photon energy.

2. The electromagnetic spectrum

3. Why particular regions are chosen for different communication systems

3.1 Microwaves – mobile phones, Wi‑Fi, satellite links

• Antenna size: Wavelengths of a few centimetres to a few millimetres allow compact “short aerials” that can be fitted on hand‑sets, routers and satellite dishes.
• Wall penetration: Microwaves are attenuated by non‑metallic walls (brick, concrete, wood) but can still reach indoor users, making them ideal for both indoor and outdoor networks.
• Bandwidth: Higher frequencies provide larger allocated bandwidths → higher data‑rate capacity (essential for 4G/5G broadband).
• Atmospheric absorption: In the 2–12 GHz range atmospheric loss is modest, so signals can travel to and from geostationary satellites with modest power and modest‑size parabolic dishes.
• Examples: 900 MHz (2G), 1800 MHz (3G), 2100 MHz (4G), 3.5 GHz (5G‑mid‑band), 5 GHz (Wi‑Fi‑ac), 12 GHz (satellite downlink).

3.2 Radio waves (VHF/UHF) – Bluetooth, short‑range personal‑area networks, broadcast TV

• Frequency band around 2.4 GHz lies in the ISM (Industrial, Scientific, Medical) spectrum, which is unlicensed worldwide – cheap, simple hardware.
• Low transmit power (< 1 mW for Bluetooth) limits range to a few metres, which is desirable for personal‑area networks and reduces interference.
• Waves at these frequencies pass through most building materials with relatively little loss, giving reliable indoor connectivity.
• Spread‑spectrum and frequency‑hopping techniques allow many devices to share the same band without severe interference.

3.3 Infrared – remote controls & short‑range data links

• IR is easily generated by inexpensive LEDs and detected by photodiodes.
• Because IR does not penetrate walls, it is naturally confined to a line‑of‑sight path – a safety advantage for consumer devices.
• Typical carrier frequencies are in the tens of kilohertz (e.g., 38 kHz) modulated onto a 38‑µm (≈ 8 THz) IR carrier.

3.4 Visible & near‑infrared – optical fibre communication

• Silica glass is extremely transparent between 800 nm and 1600 nm, giving attenuation as low as 0.2 dB km⁻¹.
• Low loss → signals can travel tens of kilometres without repeaters; for longer spans repeaters/amplifiers are used.
• Immune to electromagnetic interference, so signal quality is excellent.
• Very high carrier frequency (≈ 200 THz) provides enormous theoretical bandwidth – the practical limit is set by the electronics and multiplexing technology (e.g., DWDM).

4. Digital versus analogue signalling

• Analogue modulation varies a carrier’s amplitude, frequency or phase continuously (e.g., AM, FM, AM‑PM). Historically used for early radio and TV.
• Digital modulation encodes information as discrete bits (0/1) using techniques such as ASK, FSK, PSK, QAM, OFDM. All modern mobile, Wi‑Fi, Bluetooth and fibre‑optic systems use digital modulation.
• Advantages of digital signalling (as required by the syllabus):

  • Higher data rates – many bits per symbol.
  • Robust error‑detecting and correcting codes (e.g., CRC, Reed‑Solomon, LDPC).
  • Easy interfacing with computers and other digital devices.
  • Reduced susceptibility to noise; a degraded signal can still be recovered if the error‑rate is within limits.

5. Health & safety – non‑ionising vs. ionising radiation

• Non‑ionising radiation (radio, microwave, IR):

  • Can cause heating (e.g., microwave ovens, high‑power radar). Mobile‑phone SAR (Specific Absorption Rate) limits are set at 2 W kg⁻¹ (UK) to avoid tissue heating.
  • Intense IR can burn skin or damage the retina.

• Ionising radiation (UV‑C, X‑rays, gamma‑rays):

  • Photon energy > 10 eV can break chemical bonds, leading to DNA damage and increased cancer risk.
  • Strict shielding (lead, concrete) and exposure limits are mandated for medical and industrial use.

• Every communication system used in the syllabus operates in the non‑ionising part of the spectrum; normal everyday exposure is well below safety limits.

6. Summary – key points to remember

1. All EM waves travel at c in vacuum; frequency and wavelength are inversely related (c = fλ).
2. Microwaves (300 MHz – 300 GHz) are favoured for mobile phones, Wi‑Fi and satellite links because they allow short antennas, moderate wall penetration and large bandwidth.
3. Radio‑wave bands around 2.4 GHz are ideal for short‑range, low‑power links such as Bluetooth and some Wi‑Fi standards.
4. Optical fibres use visible/near‑IR light, giving extremely low loss and huge data capacity over long distances.
5. Modern communications are digital; digital modulation provides higher data rates, error correction and immunity to noise.
6. Only the high‑energy parts of the spectrum (UV, X‑ray, gamma) are ionising and potentially hazardous; everyday communication devices emit non‑ionising radiation within regulated safety limits.

7. Actionable review – alignment with Cambridge IGCSE 0625 syllabus