Radio Waves (10^{3}–10^{9} Hz)

• Communication: AM/FM radio, television broadcasting, mobile phones (lower frequency bands)
• How: Oscillating circuits generate radio waves; antennas receive them
• Detection: Radio antenna + receiver circuit

Microwaves (10^{9}–10^{12} Hz)

• RADAR (RAdio Detection And Ranging): Transmitted microwaves reflect off objects; time delay gives distance
• Microwave ovens: Magnetron produces microwaves → water molecule rotation → heat
• Satellite communication: Microwaves penetrate ionosphere (unlike radio waves that reflect)
• Detection: Point contact diodes

Infrared (10^{12}–$4 \times 10^{14}$ Hz)

• Night vision: Objects emit IR based on temperature; cameras detect IR in dark environments
• Physiotherapy: IR lamps heat deep tissue for pain relief and healing
• Greenhouse effect: Glass transmits visible light (enters) but absorbs IR (trapping heat)
• Detection: Thermopile, bolometer, infrared cameras

Visible Light ($4 \times 10^{14}$–$7.5 \times 10^{14}$ Hz, 400–700 nm)

• Vision: Human eye photoreceptors sensitive to 400–700 nm
• Photosynthesis: Chlorophyll absorbs red and blue light
• Photography: Light-sensitive film/CCD detectors
• Detection: Human eye, photocell, photographic film

Ultraviolet ($7.5 \times 10^{14}$–10^{17} Hz)

• Sterilisation: UV destroys DNA of microorganisms — hospitals, water purification
• LASIK surgery: Excimer UV lasers reshape cornea precisely
• Vitamin D synthesis: Skin converts 7-dehydrocholesterol to vitamin $D_{3}$ under UV
• Detection: Photocell, UV-sensitive photographic film

X-rays (10^{16}–10^{21} Hz)

• Medical imaging: X-rays pass through soft tissue, absorbed by dense bone — chest X-rays, mammography
• CT scans: Multiple X-ray images from different angles; computer reconstructs 3D images
• Crystal diffraction: Wavelength ~0.1 nm matches inter-atomic spacings; reveals crystal structure (Bragg diffraction)
• Detection: Photographic film, scintillation detectors, Geiger counters, digital flat-panel detectors

Gamma Rays (10^{18}–10^{24} Hz)

• Cancer treatment (radiotherapy): Targeted gamma beams destroy tumour cells (gamma knife surgery)
• Sterilisation of medical equipment: High-energy gamma rays kill all microorganisms
• Nuclear medicine: Radioactive tracers emit gamma rays; PET scans use positron-emitting isotopes
• Detection: Geiger counter, ionisation chamber, scintillation detectors