• $summary_{type}$: concept
• $word_{count}$: 160

Radio waves (f < 10^9 Hz, lambda > 30 cm): produced by LC oscillators and antennas. AM radio (530-1710 kHz) modulates amplitude; FM radio (88-108 MHz) modulates frequency (better noise rejection). Radio waves can reflect off the ionosphere for long-distance communication. Microwaves (1-300 GHz, lambda = 1 mm - 30 cm): produced by klystrons and magnetrons. Key frequency: 2.45 GHz (microwave ovens — resonance of water molecule rotation). Used in radar (distance from echo timing), satellite communication (penetrate ionosphere), WiFi, and cellular networks. Infrared (300 GHz - 400 THz, lambda = 1 mm - 700 nm): emitted by all warm bodies. Used in thermal imaging, night vision, remote controls, fiber-optic communication, and greenhouse effect (CO2 and water vapor absorb and re-emit IR, warming Earth). Wien's displacement law: $lambda_{max}$(um) = 2898/T(K) determines the peak emission wavelength.