Electromagnetic Waves & Communication – Detailed Study Notes

Electromagnetic Spectrum – Overview

  • The electromagnetic (EM) spectrum arranges all EM waves in order of:

    • Decreasing wavelength ➔ Increasing frequency/energy

  • Approximate wavelength & frequency bands (values given in the slides):

    • Radio: λ≈103 m  to  100 m;  f≈104 Hz\lambda \approx 10^3\,\text{m} \;\text{to}\; 10^0\,\text{m};\; f \approx 10^4\,\text{Hz}

    • Microwave: λ≈100 m  to  10−2 m;  f≈1010 Hz\lambda \approx 10^0\,\text{m}\;\text{to}\; 10^{-2}\,\text{m};\; f \approx 10^{10}\,\text{Hz}

    • Infra-red (IR): λ≈10−2 m  to  10−5 m;  f≈1012 Hz\lambda \approx 10^{-2}\,\text{m}\;\text{to}\; 10^{-5}\,\text{m};\; f \approx 10^{12}\,\text{Hz}

    • Visible: λ≈7×10−7 m→4×10−7 m;  f≈4×1014 Hz→7.5×1014 Hz\lambda \approx 7\times10^{-7}\,\text{m}\rightarrow4\times10^{-7}\,\text{m};\; f \approx 4\times10^{14}\,\text{Hz}\rightarrow 7.5\times10^{14}\,\text{Hz}

    • Ultraviolet (UV): λ≈10−7 m;  f≈1015 Hz\lambda \approx 10^{-7}\,\text{m};\; f \approx 10^{15}\,\text{Hz}

    • X-ray: λ≈10−10 m;  f≈1017 Hz\lambda \approx 10^{-10}\,\text{m};\; f \approx 10^{17}\,\text{Hz}

    • Gamma: λ≈10−12 m;  f≈1020 Hz\lambda \approx 10^{-12}\,\text{m};\; f \approx 10^{20}\,\text{Hz}

  • Photon energy range quoted: E≈103 eV→106 eVE \approx 10^3\,\text{eV}\rightarrow10^6\,\text{eV} (higher for gamma).

Fundamental Properties of EM Waves

  • Generated by mutually oscillating electric & magnetic fields.

  • Transverse waves; do not require a medium ➔ propagate in vacuum.

  • All EM waves travel in vacuum at the speed of light: c=3×108 m s−1c = 3\times10^8\,\text{m\,s}^{-1}.

  • In air, speed ≈ cc.

  • Transport energy that can be absorbed by matter, producing heating & other effects.

Specific Regions & Their Applications

Radio Waves

  • Longest λ\lambda. Radiated by aerials.

  • Uses: radio/TV broadcasting, long-distance audio, pictures & data, astronomy, RFID.

Microwaves

  • Telecommunications (international links, TV relay) via geostationary satellites.

  • Mobile-phone networks via microwave towers & low-orbit satellites.

  • Radar detection (ships, aircraft, police speed traps).

  • Cooking: water molecules resonate & heat food.

Infra-Red (IR)

  • Detected by temperature-sensitive photographic film ➔ photography in dark.

  • Satellite & aircraft sensors: weather forecasting, land-use monitoring, heat-loss audits, intruder alarms, locating earthquake victims.

  • IR lamps: rapid paint-drying (e.g.26 car finishes).

  • Remote-control transmitters for TVs/DVDs.

  • Optical-fibre pulses in telephone & data networks (IR light).

Visible Light

  • Enables sight, colour perception.

  • Employed in cameras, bulbs, photosynthesis (plants), general illumination.

Ultraviolet (UV)

  • Biological effects: tanning, vitamin-D production, risk of skin cancer.

  • Darker skin absorbs more UV.

  • Protection: hats, clothing, sunscreen.

  • Uses: security inks/signatures on bank docs, fake-note detection, water sterilisation, security marking.

X-Rays

  • Produced when high-speed eadlectrons decelerate at metal target in X-ray tube.

  • Medical imaging.

  • Airport security scanners (luggage, passenger screening).

  • Industrial weld inspection.

  • Equipment shielded with lead; high doses kill cells, lower doses can induce cancer.

Gamma Rays

  • Even more penetrating/dangerous than X-rays.

  • Cancer radiotherapy; sterilising food & surgical instruments; destroying microbes.

Health Hazards of Excessive Exposure

  • Microwaves\text{Microwaves}: internal heating of body cells.

  • Infra-red\text{Infra-red}: skin burns.

  • Ultraviolet\text{Ultraviolet}: surface-cell & eye damage (cataracts, retinal injury), skin cancer.

  • X-ray & Gamma\text{X-ray \& Gamma}: cellular mutation, DNA damage.

EM Waves in Satellite & Terrestrial Communication

  • Communication with satellites primarily uses microwaves:

    • Low-orbit satellites ➔ some satellite phones.

    • Geostationary satellites ➔ other satellite phones & direct-broadcast TV.

  • Key civilian systems:

    • Mobile phones/Wi-Fi: microwaves; penetrate some walls, require short aerials.

    • Bluetooth/radio: lower-frequency radio waves; pass through walls but attenuate.

    • Optical fibres: visible/IR light; glass transparent, high data rates.
      • Transmitter: LED or laser diode encodes electrical signal into light.
      • Receiver: photodiode decodes back to electronics.

Encoding & Decoding

  • Generic to computing, data comms, programming, digital electronics.

  • Encoding: convert letters/numbers/symbols ➔ specialised format for storage/transmission.

  • Decoding: reverse process.

  • Signals travel along wires, fibres or radio links from encoder ➔ decoder.

Analogue vs Digital Signals

Definitions

  • Analogue: continuous variation; represented by sine waves.

  • Digital: discrete levels; represented by square waves (binary 00 / 11).

Examples

  • Analogue: human voice, natural sounds, legacy electronic devices.

  • Digital: computers, optical drives, mobile phones, all modern electronics.

Waveform Sketches (described)

  • Analogue: smooth curve varying with time.

  • Digital: flat levels with abrupt transitions.

Signal Quality & Attenuation

  • All signals lose power along a path (attenuation) & collect noise.

  • Analogue amplification boosts both signal & noise ➔ degraded quality.

  • Digital pulses can be regenerated:

    • Regenerators re-shape & re-time pulses, removing accumulated noise.

Advantages of Digital

  • Cheaper circuitry.

  • Negligible distortion after regeneration.

  • Higher data rates.

  • Greater range owing to accurate, repeated regeneration.

  • Preferred for computing & consumer electronics; analogue still used for some audio/video contexts where full continuity is beneficial.