Frequency = number of cycles per second of a periodic wave. Unit: HERTZ (Hz), named after Heinrich Hertz who demonstrated electromagnetic waves in 1886. 1 Hz = 1 cycle/second. Multiples: 1 kHz = 1,000 Hz; 1 MHz = 1,000,000 Hz; 1 GHz = 1,000,000,000 Hz; 1 THz = 10¹² Hz. Wavelength: λ = c/f, where c is the wave's speed and f is frequency. For electromagnetic waves in vacuum: c = 3 × 10⁸ m/s; for sound in air at room temperature: ~340 m/s. Higher frequency = shorter wavelength. Common frequencies: (1) HOUSEHOLD AC POWER: 60 Hz (US), 50 Hz (much of world); (2) HUMAN HEARING: 20 Hz - 20 kHz (declines with age); audio frequencies; (3) AM RADIO: 535-1605 kHz (medium wave); (4) FM RADIO: 88-108 MHz; (5) TV: VHF and UHF bands; (6) WIFI: 2.4 GHz, 5 GHz, 6 GHz (Wi-Fi 6E); (7) CELL PHONES: 700 MHz - 3.5 GHz typical; (8) GPS: ~1.5 GHz; (9) BLUETOOTH: 2.4 GHz; (10) MICROWAVES (oven): 2.45 GHz; (11) X-RAYS: 10¹⁶-10¹⁹ Hz; (12) GAMMA RAYS: above 10¹⁹ Hz. The ELECTROMAGNETIC SPECTRUM (lowest to highest frequency): radio waves → microwaves → infrared → visible light (red to violet) → ultraviolet → X-rays → gamma rays. Visible light is a tiny slice of the spectrum: ~430-770 THz, corresponding to wavelengths 400-700 nm. ASVAB EI may ask about RADIO bands and applications (AM, FM, VHF, UHF, microwave). AM vs FM: AM (Amplitude Modulation) varies wave amplitude with signal — older, longer range, more susceptible to noise; FM (Frequency Modulation) varies wave frequency with signal — higher quality, less noise, shorter range.
Sideband: in AM, audio signal creates upper and lower sidebands around the carrier frequency; SSB (single-sideband) uses only one for efficiency. Bandwidth: range of frequencies a signal or system occupies.