17.1 Introduction
17.2 Radio Communications
5.3 Reflection & Refraction
5.4 Huygens' Construction
5.5 Superposition of Waves
5.6 Stationary Waves
5.7 Interference & Diffraction
5.8 Sound Waves
5.9 Doppler Effect
5.10 Electromagnetic Waves
5.11 Polarization

17.1 Introduction

- Revolution in communications since Alexxander Bell in 1876.
- Now we have satellite & radio communicaations + internet.
- How do we transmit infomation so easilyy?
- How does radio & telephone work?

17.2 Radio Communication

- Human sound communication => voice (traansmitter) & ears (receiver) - this sound doesn't carry far & many voices at once is confusing.
Radio waves - carry information much further at electromagnetic (EM) waves - 3 x 10⁸ m/s (vacuum).

Part of the EM spectrum

LW - long wave, MW - medium wave } broadcast radio
SW - short wave } broadcast public service & ammateur radio
VHF } FM radio, public
UHF } TV
Microwave - telephone micro-wave links, satellite

Radio waves - 10 kHz to 1000 MHz, human hearing - 20 Hz to 20,000 Hz.

The Transmitted Signal
- Communication systems (radio waves) traansmit information (sound, video, pictures, data)
- 2 signals make up - radio signal - 1. cconstant amplitude & frequency - carrier wave }
- 2nd signal carried inside 1st - 2. infoormation signal________________________.} modulation (mod.)
- information signal varies with differennt information________________________}

The Modulation Process
Several different ways - Morse code => on/off signals
_________________- Speech or music => more complex - 2 methods (Amplitude mod.(AM), frequency mod.(FM))

Amplitude Modulation

Information signal with carrier wave => Amplitude of carrier altered. Amplitude Modulation radio transmitter/receiver:

The Modulation Signal
Complex - mixture. Transmitter sends simple sound signal of single frequency, f0. RF carrier, fc (fc >> f0), mixer => 2 new frequencies fc + f0 & fc - f0. And mixer => f0 (audio), fc, fc + f0, fc - f0 (last 3 - radio)

5.2 Description of Waves ( examples )

Wave profile:__wavelength ( l ), time period ( T ), phase relationship ( f ) - { between particles => time interval between particle maximums (y-axis) - as a fraction of period T. }

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5.3 Reflection & Refraction ( examples )

Phase changes on reflection: wave reflected at boundary going into denser medium ( v_dense < v₁ ), or phase change at barrier is 1/2 a wavelength on reflection.

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5.4 Huygens' Construction ( examples )

- Predicts future position of a wavefrontt. Principle - "every point on a wavefront can be regarded as a source of secondary, spherical wavelets and the new wavefront will be the surface which touches all secondary wavelets."

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5.5 Superposition of Waves ( examples )____ ( web simulation )

- When 2 waves meet they produce disturbances at the points where they overlap, this is called superposition (as a resultant vector).

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Interference - 2 periodic waves of equal frequency & amplitude meet at a point. The resultant disturbance µ phase difference.
Disturbance in phase = 2 x individual waves (constructive interference)
Disturbance out of phase (180°) = zero resultant (destructive interference)
Phase difference depends on : 1) initial phase of both waves. 2) path difference - the distances travelled by the 2 waves.

Beats - 2 waves of ' » ' equal frequency meet at a point Þ varying phase difference between them.

Resultant frequancy,
Beats are variations in the amplitude from the resultant superposition (1 beat cycle => after one wave completes one more oscillation than the other).

Beat frequency, ____f₁ > f₂

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5.6 Stationary Waves ( examples )

A stationary wave is produced when 2 progressive waves (= frequency, amplitude) travelling in opposite directions overlap. This can be demonstrated from the resultant of 2 wave trains passing over each other.

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Table - Stationary vs Progressive Waves