Electromagnetic Waves

6.1 The EM Spectrum

Maxwell (1865) theoretically predicted EM waves — oscillating electric and magnetic fields propagating at speed c = 3 × 10⁸ m/s.

Hertz (1887) experimentally produced radio waves, confirming Maxwell's prediction.

Region	Wavelength	Frequency	Key Applications
Radio waves	>1 mm	<300 GHz	AM/FM radio, TV broadcast, satellite communication
Microwaves	1 mm – 1 m	300 MHz – 300 GHz	Microwave oven (2.45 GHz), mobile phones (800–2600 MHz), Wi-Fi, radar, GPS
Infrared (IR)	700 nm – 1 mm	300 GHz – 430 THz	Heat transfer, night-vision cameras, TV remotes, thermal imaging
Visible light	400–700 nm	430–770 THz	Vision, photography, solar panels
Ultraviolet (UV)	10–400 nm	770 THz – 30 PHz	Sterilisation (UV-C), vitamin D synthesis, forensics, counterfeit detection
X-rays	0.01–10 nm	30 PHz – 30 EHz	Medical imaging, airport security, crystallography
Gamma rays	<0.01 nm	>30 EHz	Cancer therapy, nuclear processes, sterilisation of food

6.2 Microwave Applications

Microwave oven: Uses 2.45 GHz microwaves — this frequency matches the resonant frequency of water molecules, causing them to rotate and generate heat internally (volumetric heating). No ionising radiation involved.

Radar (Radio Detection and Ranging): Pulses of microwave/radio waves are transmitted; echoes from objects give range (time of flight × c/2) and speed (Doppler shift).

Applications:

• Aviation (air traffic control)
• Weather forecasting (Doppler radar detects storm intensity)
• Military and speed guns

GPS (Global Positioning System): US-owned constellation of 24+ satellites transmitting microwave signals. Receivers calculate position by measuring arrival time differences from 4+ satellites (triangulation).

NavIC (Navigation with Indian Constellation): India's own navigation system.

• 7 satellites (2018); covers India + 1,500 km surroundings
• Accuracy < 20 m

6.3 Infrared Applications

• Thermal imaging cameras: Detect IR emitted by warm objects — used in night-vision goggles, search-and-rescue, fever screening (FLIR cameras), energy auditing of buildings.
• IR spectroscopy: Identifies chemical compounds by their unique absorption frequencies in the IR range.
• Remote sensing: Earth observation satellites (RESOURCESAT, CARTOSAT) use near-IR and thermal-IR bands to monitor vegetation, water bodies, and land temperature.

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