Communications & Digital Signals
Why your phone works better than your grandparents' radio
The Hook: Your Phone's Secret
When you send a selfie from London to Sydney, it travels 17,000 km in under a second. How? It bounces off satellites, shoots through optical fibres as pulses of light, and arrives perfectly clear despite all the noise along the way. Old analogue radios would sound like a crackling mess over that distance. Why does digital win?
Part 1: Satellites & The Atmosphere Problem
Baby Steps: Why Not All Waves Work for Satellites
Step 1: Earth's atmosphere has layers. The Ionosphere (50-600 km up) contains charged particles.
Step 2: Radio waves bounce off the Ionosphere like a mirror. Great for radio stations (signals travel around Earth's curve), terrible for satellites (signal reflects back down).
Step 3: Microwaves have shorter wavelengths. They pierce through the Ionosphere like a bullet through paper. Perfect for satellite links.
Interactive: Satellite Link Simulator
🔴 Radio waves reflect off the Ionosphere. Great for AM radio, useless for satellites.
Two Types of Communication Satellites
Geostationary Orbit
Altitude: 36,000 km above equator
Speed: Matches Earth's rotation (stays above one point)
Use: TV broadcasting, weather monitoring
✓ Dish points at one spot forever
Low Earth Orbit
Altitude: 500-2,000 km
Speed: Fast orbit (moves across sky)
Use: Mobile phones, internet (Starlink)
✓ Less signal delay (lag)
Part 2: Why Different Tech Uses Different Waves
The Logic: Match the Wave to the Job
Optical Fibres (IR/Visible): Highest data rate. Glass is transparent to these wavelengths. Used for internet backbone.
Microwaves (Mobile/Wi-Fi): Penetrate walls. Allow short aerials (compact devices). Medium data rate.
Radio Waves (Bluetooth/FM): Pass through obstacles but signal weakens. Lower data rate but good range.
Interactive: Tech Chooser
Technology: Optical Fibres
Wave Type: Infrared/Visible Light
Why? Extremely high data rate & glass is transparent to these wavelengths
Part 3: Digital vs Analogue Signals (The Game Changer)
Baby Steps: What's the Difference?
Analogue Signal:
A continuous wave that can have ANY value at any time. Like a smooth sine wave. Your voice is analogue (infinite variations in pitch and volume).
Digital Signal:
A discrete signal with only TWO values: HIGH (1) or LOW (0). Like a square wave. Computers speak digital (on/off, yes/no).
Interactive: The Regeneration Magic
Analogue (Sine Wave)
Digital (Square Wave)
The Magic of Regeneration:
Digital signals can be regenerated (cleaned up) because we know the signal was meant to be either 1 or 0. Analogue signals amplify the noise because we don't know what the original value was.
Benefits of Digital Transmission
✓ Clearer Signal: Noise can be removed by regeneration.
✓ Longer Range: Signal can be regenerated at relay stations without quality loss.
✓ Higher Data Rate: More information packed into the same bandwidth (e.g., video streaming).
⚠️ Examiner's Traps
Trap 1: "Radio waves are used for satellites." ❌ WRONG. Radio waves BOUNCE off the Ionosphere. Satellites use MICROWAVES.
Trap 2: "Digital signals have no noise." ❌ WRONG. Digital signals GET noise, but it can be REMOVED by regeneration.
Trap 3: "Optical fibres use radio waves." ❌ WRONG. They use INFRARED or VISIBLE LIGHT (higher frequency, higher data rate).
Trap 4: Confusing Geostationary (high orbit, fixed position) with Low Earth Orbit (low orbit, moves across sky).
📋 Syllabus Checklist (3.3 Supplement)
🎓 Summary: The Communication Revolution
Modern communication works because we've matched the RIGHT wave to the RIGHT job, and switched from analogue to digital. Microwaves penetrate the atmosphere for satellites. Optical fibres carry light at incredible data rates. And digital signals can be regenerated perfectly, no matter how much noise they pick up along the way. That's why your video call from London to Sydney is crystal clear, while your grandparents' radio crackled with static.
Sign In to Keep Going
You've reached the end of the free preview. Sign in — and upgrade to Plus or Pro — to keep going with Physics.
Full Access
Unlock all 106 lessons