Ch 15 Phy Electromagnetic spectrum

Chapter 15: Electromagnetic spectrum

15.1 Electromagnetic Waves

Discovery of Infrared Radiation 🌑

  • Heating effect strongest for red light, smallest for violet.

  • Beyond red, thermometer reading increased more β†’ invisible radiation exists β†’ Infrared radiation (IR).

  • IR = thermal radiation; heat from any hot object.

Radiation & Temperature πŸ”₯

  • Connection: cool objects β†’ low-frequency radiation; hot objects β†’ high-frequency radiation

  • Sun: surface ~5500Β°C β†’ emits UV, mostly absorbed by ozone layer

  • Ozone depletion β†’ more UV reaches Earth β†’ risk of skin cancer

Electromagnetic Waves Theory 🌊

  • Prism spectrum β†’ violet refracted most β†’ UV refracted more; IR less.

  • Light as waves like sound.

  • Frequency determines colour: red low, violet high.

  • James Clerk Maxwell: light = oscillations in electric & magnetic fields β†’ electromagnetic waves

  • Waves can have any frequency β†’ beyond IR & UV β†’ complete electromagnetic spectrum

Properties of EM Waves ⚑

  • All transverse waves

  • Travel at speed of light (~3 Γ— 10⁸ m/s in vacuum)

  • Can be reflected, refracted, diffracted

  • Frequency β†’ affects interaction with materials

Wavelength & Frequency πŸ“

  • Red light: longer wavelength, lower frequency β†’ "lazy waves"

  • Violet light: shorter wavelength, higher frequency β†’ "fast waves"

  • Visible light: 400 nm (violet) β†’ 700 nm (red), frequency ~10¹⁴ Hz

  • EM waves obey v = f Ξ» β†’ higher frequency = shorter wavelength

Uses of EM Waves πŸ“‘

Type

Uses

Radio waves

Radio/TV broadcasting, radio astronomy, RFID chips for data storage

Microwaves

Satellite TV, mobile phones, microwave ovens

Infrared (IR)

Remote controls, grills, toasters, optical fibres, medical imaging & therapy

Visible light

Vision, cameras, microscopes, photosynthesis

Ultraviolet (UV)

Forensics (body fluids), banknote security, water sterilisation

X-rays

Airport security scanners, medical imaging

Gamma rays

Cancer treatment, sterilisation, cancer detection

15.2 Electromagnetic Hazards ⚠

  • All EM radiation can be hazardous

  • IR β†’ burns

  • UV β†’ skin/eye damage, skin cancer

  • Sunbeds β†’ risk of skin damage

  • High frequency waves (X-rays & gamma rays) β†’ cell mutations, cancer

  • Protection: distance, shielding (metal cases), warning labels

  • Microwaves β†’ heating effect; caution for workers near masts & ovens

  • Mobile phones β†’ only slight heating; prolonged use β†’ potentially higher risk, especially in children

15.3 Communicating Using Electromagnetic Waves 🌐

Satellites πŸ›°

  • Orbit Earth β†’ transmit information using microwaves

  • Geostationary satellites: ~35,000 km, stay above fixed point, suitable for TV & satellite phones, slight delay

  • Low Earth Orbit (LEO): 2000 km, orbit ~2 hours, fast communication, need many satellites, less data per satellite

Right Wave for the Job πŸ“Ά

  • Mobile phones / Wi-Fi β†’ microwaves (pass walls, small aerial)

  • Bluetooth β†’ radio waves, short range, weakened by walls

  • Optical fibres β†’ IR & visible light, high frequency β†’ carry more data, faster internet

Analogue vs Digital Signals πŸ’»

  • Analogue: continuous signal, varies like sound wave, can distort β†’ noise

  • Digital: pulses (on/off), clearer, faster, compatible with computers

  • Digital call process:

    1. Sound β†’ analogue β†’ converter β†’ digital pulses

    2. Travel via optical fibres

    3. Regenerators clean & boost signal

    4. Second converter β†’ digital β†’ analogue β†’ sound

  • Digital signals β†’ high speed, accurate, long distance transmission without distortion

βœ… Key Formulas & Concepts

  • v = f Ξ» (speed = frequency Γ— wavelength)

  • Higher frequency β†’ shorter wavelength

  • Hotter object β†’ emits higher frequency radiation

  • EM spectrum: radio β†’ microwaves β†’ IR β†’ visible β†’ UV β†’ X-rays β†’ gamma

  • Random monkeys invade villagers using x-ray guns.